go/types: rename types.context to types.environment
Now that we have a Context type the context (unexported) type is
particularly confusing. Rename it to environment.
Change-Id: I7d280439b8263d9ebfd561fc4d59c6d43c8d3e3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/363176
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>