cmd/compile/internal/types2: more detailed error messages for generic conversions

- slightly refactor convertibleTo and convertibleToImpl
- provide ability to return a conversion failure cause
- add detailed cause for generic conversions

For #47150.

Change-Id: Ie97d89be0234414ef4df22a6920e18acc944a102
Reviewed-on: https://go-review.googlesource.com/c/go/+/357249


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