[dev.typeparams] go/types: use the TParams API consistently

Even internally to the type checker, we should use the TParams and
RParams methods instead of accessing fields directly, as TParams may be
lazily expanded, and in the future we may want to pack receiver and
function type parameters into a single field on Signature.

We should also not differentiate a nil *TParamList from an empty
*TParamList.

Change-Id: I85c616e6c708a89b6a5eb1e69fe0b014276eda90
Reviewed-on: https://go-review.googlesource.com/c/go/+/336251


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