spec: add type parameter types

- add section on type parameters
- added two sections on the scope of type parameters
- expanded general section on types accordingly
- introduced the notion of a named type which will
  help in simplifying various rules (subsequent CLs)

Change-Id: I49c1ed7d6d4f951d751f0a3ca5dfb637e49829f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/365414


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>