[wasi-common]: move filetime module to yanix (#1255)

* Move filetime module to yanix

I've noticed that we could replace every occurrence of `crate::Result`
in `filetime` mods with `io::Result`, so I thought why not move it
to `yanix` and get rid off a lot of unnecessary code duplication
within `wasi-common`. Now, ideally I'd have our `filetime` modifications
backported to Alex's [`filetime`] crate, but one step at a time
(apologies Alex, I was meant to backport this ages ago, just didn't
find the time yet... :-().

Anyway, this commit does just that; i.e., moves the `filetime` modules
into `yanix` which seems a better fit for this type of code.

[`filetime`]: https://github.com/alexcrichton/filetime

There is one caveat here. On Emscripten, converting between `filetime::Filetime`
and `libc::timespec` appears to be lossy, at least as far as the
types are concerned. Now, `filetime::Filetime`'s seconds field is
`i64` while nanoseconds field is `u32`, while Emscripten's
`libc::timespec` requires both to be `i32` width. This might actually
not be a problem since I don't think it's possible to fill `filetime::Filetime`
struct with values of width wider than `i32` since Emscripten is 32bit
but just to be on the safe side, we do a `TryInto` conversion, log
the error (if any), and return `libc::EOVERFLOW`.

* Run cargo fmt

* Use i64::from instead of as cast

Cargo.lock

@@ -2605,6 +2605,7 @@ version = "0.12.0"
 dependencies = [
  "bitflags",
  "cfg-if",
+ "filetime",
  "libc",
  "log",
 ]