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/unix/stdio.rs

@@ -0,0 +1,59 @@
+use super::{get_file_type, get_rights};
+use crate::handle::Handle;
+use crate::sys::stdio::{Stderr, StderrExt, Stdin, StdinExt, Stdout, StdoutExt};
+use std::cell::Cell;
+use std::fs::File;
+use std::io;
+use std::mem::ManuallyDrop;
+use std::os::unix::prelude::{AsRawFd, FromRawFd, RawFd};
+
+impl AsRawFd for Stdin {
+    fn as_raw_fd(&self) -> RawFd {
+        io::stdin().as_raw_fd()
+    }
+}
+
+impl AsRawFd for Stdout {
+    fn as_raw_fd(&self) -> RawFd {
+        io::stdout().as_raw_fd()
+    }
+}
+
+impl AsRawFd for Stderr {
+    fn as_raw_fd(&self) -> RawFd {
+        io::stderr().as_raw_fd()
+    }
+}
+
+impl StdinExt for Stdin {
+    fn stdin() -> io::Result<Box<dyn Handle>> {
+        let file = unsafe { File::from_raw_fd(io::stdin().as_raw_fd()) };
+        let file = ManuallyDrop::new(file);
+        let file_type = get_file_type(&file)?;
+        let rights = get_rights(&file, &file_type)?;
+        let rights = Cell::new(rights);
+        Ok(Box::new(Self { file_type, rights }))
+    }
+}
+
+impl StdoutExt for Stdout {
+    fn stdout() -> io::Result<Box<dyn Handle>> {
+        let file = unsafe { File::from_raw_fd(io::stdout().as_raw_fd()) };
+        let file = ManuallyDrop::new(file);
+        let file_type = get_file_type(&file)?;
+        let rights = get_rights(&file, &file_type)?;
+        let rights = Cell::new(rights);
+        Ok(Box::new(Self { file_type, rights }))
+    }
+}
+
+impl StderrExt for Stderr {
+    fn stderr() -> io::Result<Box<dyn Handle>> {
+        let file = unsafe { File::from_raw_fd(io::stderr().as_raw_fd()) };
+        let file = ManuallyDrop::new(file);
+        let file_type = get_file_type(&file)?;
+        let rights = get_rights(&file, &file_type)?;
+        let rights = Cell::new(rights);
+        Ok(Box::new(Self { file_type, rights }))
+    }
+}