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.