Add a way to typecheck a Value or interface{} without converting it to
the other form. This permits implementations which store field values as
a Value (such as dynamicpb, or (soon) extensions in generated messages)
to validate inputs without an unnecessary conversion.
Fixesgolang/protobuf#905
Change-Id: I1b78612b22ae832efbb55f81ae420871729e3a02
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/192457
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>