Offset cell variables from the base of locals

[ericsnowcurrently]

( from #35)

• update the compiler
• update PyFrameObject & related code
• update the eval loop

(See python/cpython#25152. Raymond has some concerns in https://bugs.python.org/issue43693.)

[ericsnowcurrently]

Per Guido, I'm keeping the LOAD_CLOSURE opcode around for readability.

[ericsnowcurrently]

done