Introduce strongly-typed system primitives (#1561)

* Introduce strongly-typed system primitives

This commit does a lot of reshuffling and even some more. It introduces
strongly-typed system primitives which are: `OsFile`, `OsDir`, `Stdio`,
and `OsOther`. Those primitives are separate structs now, each implementing
a subset of `Handle` methods, rather than all being an enumeration of some
supertype such as `OsHandle`. To summarise the structs:

* `OsFile` represents a regular file, and implements fd-ops
  of `Handle` trait
* `OsDir` represents a directory, and primarily implements path-ops, plus
  `readdir` and some common fd-ops such as `fdstat`, etc.
* `Stdio` represents a stdio handle, and implements a subset of fd-ops
  such as `fdstat` _and_ `read_` and `write_vectored` calls
* `OsOther` currently represents anything else and implements a set similar
  to that implemented by `Stdio`

This commit is effectively an experiment and an excercise into better
understanding what's going on for each OS resource/type under-the-hood.
It's meant to give us some intuition in order to move on with the idea
of having strongly-typed handles in WASI both in the syscall impl as well
as at the libc level.

Some more minor changes include making `OsHandle` represent an OS-specific
wrapper for a raw OS handle (Unix fd or Windows handle). Also, since `OsDir`
is tricky across OSes, we also have a supertype of `OsHandle` called
`OsDirHandle` which may store a `DIR*` stream pointer (mainly BSD). Last but not
least, the `Filetype` and `Rights` are now computed when the resource is created,
rather than every time we call `Handle::get_file_type` and `Handle::get_rights`.
Finally, in order to facilitate the latter, I've converted `EntryRights` into
`HandleRights` and pushed them into each `Handle` implementor.

* Do not adjust rights on Stdio

* Clean up testing for TTY and escaping writes

* Implement AsFile for dyn Handle

This cleans up a lot of repeating boilerplate code todo with
dynamic dispatch.

* Delegate definition of OsDir to OS-specific modules

Delegates defining `OsDir` struct to OS-specific modules (BSD, Linux,
Emscripten, Windows). This way, `OsDir` can safely re-use `OsHandle`
for raw OS handle storage, and can store some aux data such as an
initialized stream ptr in case of BSD. As a result, we can safely
get rid of `OsDirHandle` which IMHO was causing unnecessary noise and
overcomplicating the design. On the other hand, delegating definition
of `OsDir` to OS-specific modules isn't super clean in and of itself
either. Perhaps there's a better way of handling this?

* Check if filetype of OS handle matches WASI filetype when creating

It seems prudent to check if the passed in `File` instance is of
type matching that of the requested WASI filetype. In other words,
we'd like to avoid situations where `OsFile` is created from a
pipe.

* Make AsFile fallible

Return `EBADF` in `AsFile` in case a `Handle` cannot be made into
a `std::fs::File`.

* Remove unnecessary as_file conversion

* Remove unnecessary check for TTY for Stdio handle type

* Fix incorrect stdio ctors on Unix

* Split Stdio into three separate types: Stdin, Stdout, Stderr

* Rename PendingEntry::File to PendingEntry::OsHandle to avoid confusion

* Rename OsHandle to RawOsHandle

Also, since `RawOsHandle` on *nix doesn't need interior mutability
wrt the inner raw file descriptor, we can safely swap the `RawFd`
for `File` instance.

* Add docs explaining what OsOther is

* Allow for stdio to be non-character-device (e.g., piped)

* Return error on bad preopen rather than panic

crates/wasi-common/src/sys/mod.rs

@@ -1,6 +1,9 @@
 pub(crate) mod clock;
 pub(crate) mod fd;
-pub(crate) mod oshandle;
+pub(crate) mod osdir;
+pub(crate) mod osfile;
+pub(crate) mod osother;
+pub(crate) mod stdio;
 
 use cfg_if::cfg_if;
 
@@ -20,3 +23,65 @@ cfg_if! {
 
 pub(crate) use sys_impl::path;
 pub(crate) use sys_impl::poll;
+
+use super::handle::Handle;
+use crate::wasi::types;
+use osdir::OsDir;
+use osfile::OsFile;
+use osother::OsOther;
+use std::convert::TryFrom;
+use std::fs::File;
+use std::io;
+use std::mem::ManuallyDrop;
+use stdio::{Stderr, Stdin, Stdout};
+use sys_impl::get_file_type;
+
+pub(crate) trait AsFile {
+    fn as_file(&self) -> io::Result<ManuallyDrop<File>>;
+}
+
+impl AsFile for dyn Handle + 'static {
+    fn as_file(&self) -> io::Result<ManuallyDrop<File>> {
+        if let Some(file) = self.as_any().downcast_ref::<OsFile>() {
+            file.as_file()
+        } else if let Some(dir) = self.as_any().downcast_ref::<OsDir>() {
+            dir.as_file()
+        } else if let Some(stdin) = self.as_any().downcast_ref::<Stdin>() {
+            stdin.as_file()
+        } else if let Some(stdout) = self.as_any().downcast_ref::<Stdout>() {
+            stdout.as_file()
+        } else if let Some(stderr) = self.as_any().downcast_ref::<Stderr>() {
+            stderr.as_file()
+        } else if let Some(other) = self.as_any().downcast_ref::<OsOther>() {
+            other.as_file()
+        } else {
+            log::error!("tried to make std::fs::File from non-OS handle");
+            Err(io::Error::from_raw_os_error(libc::EBADF))
+        }
+    }
+}
+
+impl TryFrom<File> for Box<dyn Handle> {
+    type Error = io::Error;
+
+    fn try_from(file: File) -> io::Result<Self> {
+        let file_type = get_file_type(&file)?;
+        match file_type {
+            types::Filetype::RegularFile => {
+                let handle = OsFile::try_from(file)?;
+                log::debug!("Created new instance of OsFile: {:?}", handle);
+                Ok(Box::new(handle))
+            }
+            types::Filetype::Directory => {
+                let handle = OsDir::try_from(file)?;
+                log::debug!("Created new instance of OsDir: {:?}", handle);
+                Ok(Box::new(handle))
+            }
+            _ => {
+                let handle = OsOther::try_from(file)?;
+                log::debug!("Created new instance of OsOther: {:?}", handle);
+                Ok(Box::new(handle))
+            }
+        }
+    }
+}