Unmarshaling of scalar, messages, repeated, and maps.
Need to further improve on error messages for consistency, some error
messages contain the position info while some currently do not. There
are cases where position info is wrong as well when a value is decoded
in another pass, e.g. numbers in string value, or map keys.
Change-Id: I6f9e903c499b5e87fb258dbdada7434389fc7522
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/166338
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
