This renames the internal writefs package to syscallfs as it is largely
dependent on syscall signatures. This also implements utimes in gojs.
WASI will be a follow-up change as it requires more infrastructure.
Notably, we also need non-TinyGo tests because TinyGo doesn't yet
support os.Chtimes or corresponding syscalls.
Signed-off-by: Adrian Cole <adrian@tetrate.io>