This adds true GOOS=wasip1 support in addition to our existing
-target=wasi support. The old support for WASI isn't removed, but should
be treated as deprecated and will likely be removed eventually to reduce
the test burden.
1.15 specific files deleted.
1.16 specific files folded carefully into generic files, with goal of reducing diff with upstream.
Follows upstream 1.16 in making PathError etc. be aliases for the same errors in io/fs.
This fixes#2817 and lets us add io/ioutil to "make test-tinygo" on linux and mac.
Ayke van Laethem
Yurii Soldak
Dan Kegel