go/types, types2: rename structuralType/String to coreType/String

This is a pure rename of the respective Go functions/methods
with corresponding adjustments to error messages and tests.
A couple of comments were manually rephrased.

With this change, the implementation and error messages match
the latest spec.

No functionality change.

Change-Id: Iaa92a08b64756356fb2c5abdaca5c943c9105c96
Reviewed-on: https://go-review.googlesource.com/c/go/+/384618


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