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/fd.rs

@@ -1,4 +1,6 @@
-use super::oshandle::OsFile;
+use super::oshandle::RawOsHandle;
+use crate::sys::osdir::OsDir;
+use crate::sys::osfile::OsFile;
 use crate::wasi::{self, types, Result};
 use std::convert::TryInto;
 use std::fs::File;
@@ -9,7 +11,7 @@ pub(crate) fn fdstat_get(fd: &File) -> Result<types::Fdflags> {
     Ok(fdflags.into())
 }
 
-pub(crate) fn fdstat_set_flags(fd: &File, fdflags: types::Fdflags) -> Result<Option<OsFile>> {
+pub(crate) fn fdstat_set_flags(fd: &File, fdflags: types::Fdflags) -> Result<Option<RawOsHandle>> {
     unsafe { yanix::fcntl::set_status_flags(fd.as_raw_fd(), fdflags.into())? };
     // We return None here to signal that the operation succeeded on the original
     // file descriptor and mutating the original WASI Descriptor is thus unnecessary.
@@ -45,14 +47,14 @@ pub(crate) fn filestat_get(file: &File) -> Result<types::Filestat> {
 }
 
 pub(crate) fn readdir<'a>(
-    file: &'a OsFile,
+    dirfd: &'a OsDir,
     cookie: types::Dircookie,
 ) -> Result<Box<dyn Iterator<Item = Result<(types::Dirent, String)>> + 'a>> {
     use yanix::dir::{DirIter, Entry, EntryExt, SeekLoc};
 
     // Get an instance of `Dir`; this is host-specific due to intricasies
     // of managing a dir stream between Linux and BSD *nixes
-    let mut dir = file.dir_stream()?;
+    let mut dir = dirfd.stream_ptr()?;
 
     // Seek if needed. Unless cookie is wasi::__WASI_DIRCOOKIE_START,
     // new items may not be returned to the caller.