Rewrite majority of impl reusing libstd (#34)

* Rewrite FdEntry reusing as much libstd as possible

* Use the new FdEntry, FdObject, Descriptor struct in *nix impl

* Adapt Windows impl

* Remove unnecessary check in fd_read

Check `host_nread == 0` caused premature FdEntry closure and removal
which ultimately was resulting in an attempt at "double closing" of
the same file descriptor at the end of the Wasm program:
...
fd_close(fd=4)
    -> errno=WASI_ESUCCESS
fd_close(fd=4)
    -> errno=WASI_EBADF

* Use libstd vectored IO

* Use std:🧵:yield_now to implement sched_yield

* Add logging to integration tests

* Add preliminary support for host-specific errors

* Operate on std::fs::File in path_get on *nix

* Add cross-platform RawString type encapsulating OsStrExt

* Fix Windows build

* Update Travis and README to Rust v1.36

* Remove unused winx::handle::close helper

* Refactor Descriptor into raw handles/fds

* Strip readlinkat in prep for path_get host-independent

* Strip openat in prep for path_get host-independent

* Move ManuallyDrop up one level from Descriptor to FdObject

* Make (c)iovec host fns unsafe

* Swap unwraps/expects for Results in fdentry_impl on nix

* Rewrite fd_pread/write and implement for Win

* Use File::sync_all to impl fd_sync

* Use File::sync_data to impl fd_datasync

* Rewind file cursor after fd_p{read, write} on Windows

* Add fd_p{read, write} tests

* Handle errors instead of panicking in path_get

* Use File::set_len to impl fd_allocate

* Add test for fd_allocate

* Replace all panics with Results

* Document the point of RawString

src/host.rs

@@ -1,9 +1,11 @@
 //! WASI host types as defined in host. This file was originally generated
 //! by running bindgen over wasi/core.h, and the content
 //! still largely reflects that.
-
 #![allow(non_camel_case_types)]
 #![allow(non_snake_case)]
+
+use std::{io, slice};
+
 pub type void = ::std::os::raw::c_void;
 
 pub type __wasi_advice_t = u8;
@@ -473,6 +475,26 @@ pub struct __wasi_subscription_t___wasi_subscription_u___wasi_subscription_u_fd_
     pub fd: __wasi_fd_t,
 }
 
+pub unsafe fn ciovec_to_host<'a>(ciovec: &'a __wasi_ciovec_t) -> io::IoSlice<'a> {
+    let slice = slice::from_raw_parts(ciovec.buf as *const u8, ciovec.buf_len);
+    io::IoSlice::new(slice)
+}
+
+pub unsafe fn ciovec_to_host_mut<'a>(ciovec: &'a mut __wasi_ciovec_t) -> io::IoSliceMut<'a> {
+    let slice = slice::from_raw_parts_mut(ciovec.buf as *mut u8, ciovec.buf_len);
+    io::IoSliceMut::new(slice)
+}
+
+pub unsafe fn iovec_to_host<'a>(iovec: &'a __wasi_iovec_t) -> io::IoSlice<'a> {
+    let slice = slice::from_raw_parts(iovec.buf as *const u8, iovec.buf_len);
+    io::IoSlice::new(slice)
+}
+
+pub unsafe fn iovec_to_host_mut<'a>(iovec: &'a mut __wasi_iovec_t) -> io::IoSliceMut<'a> {
+    let slice = slice::from_raw_parts_mut(iovec.buf as *mut u8, iovec.buf_len);
+    io::IoSliceMut::new(slice)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;