Compile wasi-common to Emscripten (#688)

* Compile wasi-common to Emscripten

This commit enables cross-compiling of `wasi-common` to Emscripten. To achieve
this, this commit does quite a bit reshuffling in the existing codebase. Namely,
* rename `linux` modules in `wasi-common` and `yanix` to `linux_like` -- this is
  needed so that we can separate out logic specific to Linux and Emscripten out
* tweak `dir` module in `yanix` to support Emscripten -- in particular, the main
  change involves `SeekLoc::from_raw` which has to be now host-specific, and is now
  fallible
* tweak `filetime` so that in Emscripten we never check for existence of `utimensat`
  at runtime since we are guaranteed for it to exist by design
* since `utimes` and `futimes` are not present in Emscripten, move them into a separate
  module, `utimesat`, and tag it cfg-non-emscripten only
* finally, `to_timespec` is now fallible since on Emscripten we have to cast number of
  seconds, `FileTime::seconds` from `i64` to `libc::c_long` which resolves to `i32`
  unlike on other nixes

* Fix macos build

* Verify wasi-common compiles to Emscripten

This commit adds `emscripten` job to Github Actions which installs
`wasm32-unknown-emscripten` target, and builds `wasi-common` crate.

* Use #[path] to cherry-pick mods for Emscripten

This commit effectively reverses the reorg introduced in 145f4a5
in that it ditches `linux_like` mod for separate mods `linux` and
`emscripten` which are now on the same crate level, and instead,
pulls in common bits from `linux` using the `#[path = ..]` proc
macro.

crates/wasi-common/src/error.rs

@@ -149,6 +149,12 @@ impl From<str::Utf8Error> for Error {
     }
 }
 
+impl From<ffi::NulError> for Error {
+    fn from(_: ffi::NulError) -> Self {
+        Self::Wasi(WasiError::EILSEQ)
+    }
+}
+
 impl From<&ffi::NulError> for Error {
     fn from(_: &ffi::NulError) -> Self {
         Self::Wasi(WasiError::EILSEQ)