go/types: use a TypeList type to hold type arguments

This resolves an asymmetry between the TParams and TArgs APIs, and
reduces the size of the Named struct at the cost of some additional nil
checks.

While at it, move TParamList and TypeList to a new file:typelists.go,
and change TParamList to access the tparams slice directly in At. There
is no reason to guard against a nil receiver, as accessing an index on
the empty slice will panic anyway.

Change-Id: I9b65247e06c697a57a4efe40c3390e0faff91441
Reviewed-on: https://go-review.googlesource.com/c/go/+/343933


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>