Refactor the fast-path size, marshal, unmarshal, and isinit functions to
take the *coderFieldInfo for the field as input.
This replaces a number of closures capturing field-specific information
with functions taking that information as an explicit parameter.
Change-Id: I8cb39701265edb7b673f6f04a0152d5f4dbb4d5d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/218937
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>