go/types, types2: use version data type instead of major,minor ints

go/types, types2: use version data type instead of major,minor ints

Also, move version type declaration and associated operations to
the top of version.go.

Change-Id: I1e6e27c58f97fb2a2ac441dcb97bb7decf8dce71
Reviewed-on: https://go-review.googlesource.com/c/go/+/491795
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
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