internal/impl: inline coderInfoFields for better cache locality
internal/impl: inline coderInfoFields for better cache locality

Microbenchmarks are inconclusive/noisy, but shows a small but noticeable
improvement on internal benchmarks.

Change-Id: Ic46c6aac8a42c4dc749c4f3583d8c8c95e9548b7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/229277
Reviewed-by: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go