py/modsys: Implement maybe_intern() function.
py/modsys: Implement maybe_intern() function.

If interned string with the same content as input argument exists, return
it. Otherwise, return the original string.

Change-Id: Id280a21fd6f982c5804bec77f22a69bb222aef06

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