go/types, types2: add more tests for unsafe.Slice/SliceData/String/StringData
Also:
- fine-tune the implementation for some of the new builtin functions
- make sure the go/types code is an exact as possible copy of the
types2 code
- fix the description and examples for errorcodes.go
Follow-up on CL 423754.
For #53003.
Change-Id: I5c70b74e90c724cf6c842cedc6f8ace26fde372b
Reviewed-on: https://go-review.googlesource.com/c/go/+/425454 Reviewed-by: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Robert Griesemer <gri@google.com>