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.

.github/workflows/main.yml

@@ -87,6 +87,19 @@ jobs:
     - run: cargo fuzz run instantiate_translated --release --debug-assertions -- -runs=0
     - run: cargo fuzz run api_calls              --release --debug-assertions -- -runs=0
 
+  # Install wasm32-unknown-emscripten target, and ensure `crates/wasi-common`
+  # compiles to Emscripten.
+  emscripten:
+    name: Emscripten
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+      with:
+        submodules: true
+    - uses: ./.github/actions/install-rust
+    - run: rustup target add wasm32-unknown-emscripten
+    - run: cargo build --target wasm32-unknown-emscripten -p wasi-common
+
   # Perform all tests (debug mode) for `wasmtime`. This runs stable/beta/nightly
   # channels of Rust as well as macOS/Linux/Windows.
   test: