go/types: spell out 'Type' in type parameter APIs

As discussed on the go/types proposal (#47916), we should spell out the
word 'Type', rather than using 'T'.

Change-Id: I5f51255eedc07fea61f909b7ecb3093a7fab765e
Reviewed-on: https://go-review.googlesource.com/c/go/+/348376


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