The fast-path unmarshal funcs return the number of bytes consumed.
Change these functions to return an unmarshalOutput struct instead, to
make it easier to add to the results. This is groundwork for allowing
the fast-path unmarshaler to indicate when the unmarshaled message is
known to be initialized.
Change-Id: Ia8c44731a88f5be969a55cd98ea26282f412c7ae
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/215720
Reviewed-by: Joe Tsai <joetsai@google.com>