goprotobuf: Remove the proto.Buffer freelist.

Benchmarks show this is an overall performance improvement.

R=r
CC=golang-codereviews
https://codereview.appspot.com/51860043
goprotobuf: Support encoding.TextMarshaler and encoding.TextUnmarshaler.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/65740044
goprotobuf: Include package docs from .proto in the generated package docs.

These kinds of comments in .proto files usually aren't there
but if they are then they are valuable to have in the generated code.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/65860043