cmd/compile/internal/types2: rename structure to structuralType
And rename structureString to structuralString.
Now that we have an updated definition for structural types in
the (forthcoming) spec, name the corresponding function accordingly.
No semantic changes.
Change-Id: Iab838f01a37075bedf2d8bc4f166b0217672b85f
Reviewed-on: https://go-review.googlesource.com/c/go/+/362994
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>