cmd/compile/internal/types2: use a TypeList type to hold type arguments

This is a port of CL 343933 from go/types with the necessary
adjustments in the compiler.

With this CL type parameters and type lists are now held in
TParamList and TypeList data types which don't expose the
internal representation.

Change-Id: I6d60881b5db995dbc04ed3f4a96e8b5d41f83969
Reviewed-on: https://go-review.googlesource.com/c/go/+/344615


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dan Scales <danscales@google.com>