cmd/compile/internal/types2: generalize assignability to generic types

Similar to conversions, handle ordinary cases first, followed by
type-by-type assignability tests in case of type parameters with
specific types in their type sets.

Add a new class of type checker tests, in testdata/spec, which
I hope we can populate over time with tests following the spec
organization. Moved the conversions.go2 tests in the same dir.

Change-Id: Iac253ae375c08022bdc39e92e3951ec3f509e432
Reviewed-on: https://go-review.googlesource.com/c/go/+/357917


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>