Stop using double underscores for our C function names and standardize on the 'rust_proto_' prefix.

Besides unnecessary inconsistency on our C symbols, double underscores anywhere in the name are reserved for stdlib use.  In practice its unlikely these symbols would ever hit a collision problem (maybe the prior name 'utf8_debug_string' with no prefix as having some risk), but safer to just standardize on this and have no concerns going forward.

PiperOrigin-RevId: 648709299