Add enough Windows functionality to run WASI tutorial (#22)

* Add partial impl of determine_type_rights fn

* Add draft of fd_fdstat_get hostcall

* Add writev wrapper for writing IoVec in RawHandle

* Move IoVec and writev to separate helper crate

* Add Win error handling

Clean up closing and duplicating RawHandle

* Wrap Win file type result

* Add draft impl of fd_close and fd_read

* Refactor getting file access rights

* Remove winapi from the main Cargo.toml

* Add very rough draft of open_path (very incomplete)

* Clean up WinError with macro

* Ignore dir handle in openat if path absolute

* Decode oflags and advance open_path hostcall

* Clean up AccessRight and FlagsAndAttributes flags

* Implement path_get (without symlink expansion yet!)

* Add ShareMode and fix path_get for nested paths

* Add some error mappings between Win and WASI

* Clean up fdflags conversions

* Fix sharing violation when calling openat at '.'

* Apply Alex's fix of using ManuallyDrop instead forget

* Clean up

* Explicitly specify workspace to avoid comp errors at tests

src/sys/windows/hostcalls_impl/mod.rs

@@ -1,6 +1,7 @@
 //! Windows-specific hostcalls that implement
 //! [WASI](https://github.com/CraneStation/wasmtime-wasi/blob/wasi/docs/WASI-overview.md).
 mod fs;
+mod fs_helpers;
 mod misc;
 
 use super::fdentry;