py/persistentcode: Add new type of bytecode const object: global reference.
py/persistentcode: Add new type of bytecode const object: global reference.

Global object name gets resolved at bytecode load time, not at runtime.
Currently, C code doesn't generate these references, it's for experiments
using bytecode assembly.

Change-Id: I0fe9e1a3ad14d506470e37f9629f42d818798ed3

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython