Auto merge of #59619 - alexcrichton:wasi-fs, r=fitzgen

wasi: Implement more of the standard library

This commit fills out more of the `wasm32-unknown-wasi` target's standard library, notably the `std::fs` module and all of its internals. A few tweaks were made along the way to non-`fs` modules, but the last commit contains the bulk of the work which is to wire up all APIs to their equivalent on WASI targets instead of unconditionally returning "unsupported". After this some basic filesystem operations and such should all be working in WASI!

src/libstd/sys_common/mod.rs

@@ -61,6 +61,7 @@ pub mod util;
 pub mod wtf8;
 pub mod bytestring;
 pub mod process;
+pub mod fs;
 
 cfg_if! {
     if #[cfg(any(target_os = "cloudabi",