cmd/compile/internal/types2: implement singleType and structure (type)

Rename structuralType to singleType throughout. This reflects
more closely what the function does: if a type set consists of
exactly one type term, singleType returns the corresponding type.

Rename singleUnder to structure. The structure function returns
the "type structure" of a type, either its underlying type for
a non-type parameter, or the single underlying type (if it exists)
for a type parameter.

Change constraint type inference to use the structure type for
inference, unless the structure type is the underlying type of
a single defined type, in which case it uses the latter. This
preserves existing behavior while making constraint type inference
slightly more flexible.

Change-Id: I38ee89ffdabd12bfeaa0be2ad6af8fb373c11fc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/359015


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