go/types: move to an opaque environment for Instantiate
To match the API proposal, switch the first argument to Instantiate to
an opaque Environment handle, though for now this handle is
unimplemented.
Change-Id: I6207f0beafdf8497587abdad37db92f927db29b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/343930
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>