Do it all at once in preparation for Go 1.18 support.
To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.
This is File.Stat from https://github.com/tinygo-org/tinygo/pull/2371,
plus the windows bits,
plus a smoke test more or less from upstream,
all pulled together and rebased by dkegel-fastly.
Add direct test for the problem to make the fix commit clearer.
Noticed while implementing MkdirTemp on mac; the upstream tests for MkdirTemp fail without this.
File.Stat is left as a stub for now.
Tests are a bit stubbed down because os.ReadDir, os.Symlink, and t.TempDir are not yet (fully) implemented.
TODO: reimport tests from upstream as those materialize.
