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

Cargo.toml

@@ -34,6 +34,7 @@ cranelift-entity = "0.33.0"
 cranelift-wasm = "0.33.0"
 cranelift-native = "0.33.0"
 target-lexicon = "0.3.0"
+pretty_env_logger = "0.3.0"
 
 [patch."https://github.com/CraneStation/wasi-common"]
 wasi-common = { path = "." }