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

build.rs

@@ -55,7 +55,7 @@ fn test_directory(out: &mut File, testsuite: &str) -> io::Result<()> {
             .expect("testsuite filename should be representable as a string")
             .replace("-", "_")
     )?;
-    writeln!(out, "    use super::{{runtime, utils}};")?;
+    writeln!(out, "    use super::{{runtime, utils, setup_log}};")?;
     for dir_entry in dir_entries {
         write_testsuite_tests(out, dir_entry, testsuite)?;
     }
@@ -80,6 +80,7 @@ fn write_testsuite_tests(out: &mut File, dir_entry: DirEntry, testsuite: &str) -
         "    fn {}() -> Result<(), String> {{",
         avoid_keywords(&stemstr.replace("-", "_"))
     )?;
+    write!(out, "        setup_log();")?;
     write!(out, "        let path = std::path::Path::new(\"")?;
     // Write out the string with escape_debug to prevent special characters such
     // as backslash from being reinterpreted.
@@ -131,6 +132,8 @@ cfg_if::cfg_if! {
             if testsuite == "misc_testsuite" {
                 match name {
                     "big_random_buf" => false,
+                    "sched_yield" => false,
+                    "file_pread_pwrite" => false,
                     _ => true,
                 }
             } else {