[wasi-common] Clean up fd_filestat_get implementation (#757)

* Clean up fd_filestat_get implementation

This commit does 4 things:
* Adds `yanix::file::fstat`, a wrapper around `libc::fstat`.
* It essentially reverts 89fbde2 for Unix hosts -- in other words,
  it brings back the use of `fstat` to obtain `libc::stat` from a
  file descriptor, rather than relying on `std::fs::Metadata`. This
  way, we reuse `host_impl::filestat_from_nix` in
  `hostcalls_impl::fd_filestat_get` implementation rather than
  unnecessarily duplicate code for converting filestats into
  `__wasi_filestat_t`.
* Moves `crate::helpers::systemtime_to_timestamp` to Windows `host_impl`
  module. It does the same thing with helpers which assist in converting
  `std::fs::Metadata` into `__wasi_filestat_t`. This should retain symmetry
  between *nix and Windows impls.
* Makes timestamp conversions in `host_impl::filestat_from_nix` fallible.

* Backport changes to snapshot0

* Signal no overflow with `from` rather than `as` cast

crates/wasi-common/src/hostcalls_impl/fs.rs

@@ -710,8 +710,7 @@ pub(crate) unsafe fn fd_filestat_get(
     );
 
     let fd = wasi_ctx.get_fd_entry(fd)?.as_descriptor(0, 0)?.as_file()?;
-
-    let host_filestat = hostcalls_impl::fd_filestat_get_impl(fd)?;
+    let host_filestat = hostcalls_impl::fd_filestat_get(fd)?;
 
     trace!("     | *filestat_ptr={:?}", host_filestat);