The code organization is simpler if we keep the functions encoding and
decoding a particular type (e.g., maps) together rather than split
across files. Rename various "encode" files to "codec" in preparation
for adding fast-path decoding.
Change-Id: If1e271da99d31533ffefc19b1fc847936fa9484a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185241
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
