[dev.typeparams] go/types: set type parameter indices when they are bound

It is invalid to use a type parameter for more than one type, so we can
avoid passing the type parameter index to NewTypeParam and just set it
when type parameters are bound to a type via SetTParams or during type
checking.

In order to enforce the correctness of this change, introduce a
TypeParams type to represent a list of type parameters that have been
associated with a type. For now, expose this new type as the API for
type parameters, but this is of course not necessarily a final API.

Allowing *TypeParams to be nil also decreases the size of Named and
Signature, which is good as most instances of these types will not be
parameterized.

Change-Id: Ia1e39ba51edb05bb535eb5f41c34e9dd02d39c38
Reviewed-on: https://go-review.googlesource.com/c/go/+/336249


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>