fix build tags and become a module

[chappjc]

The old style build tags were not working as intended due misplaced spaces.

// +build amd64, !gccgo, !appengine

In the above old style +build line, which is now replaced with //go:build lines that use a standard boolean syntax, the spaces mean OR, while commas mean AND.

When go fmt is run on the source with Go 1.17 or 1.18, it translates the above line into the following:

//go:build (amd64 && ignore) || (!gccgo && ignore) || !appengine
// +build amd64,ignore !gccgo,ignore !appengine

So this build line was not considering either the amd64 or gccgo tags, just the !appengine.

The intended build tags for siphash_amd64.go seem to be the following:

//go:build amd64 && !gccgo && !appengine && !nacl
// +build amd64,!gccgo,!appengine,!nacl

The tags for siphash_ref.go are still the following:

//go:build (!amd64 && !386) || gccgo || appengine || nacl
// +build !amd64,!386 gccgo appengine nacl

This resolves a build error with multiple definitions of core when gccgo is set.

This also makes github.com/aead/siphash into a Go module.