Add mapping for automatic parallelization over inputs

[jlowin]

This exposes a map utility as an attribute of the major Marvin functions cast, extract, and classify.

Mapping is performed by calling each function's "map" attribute, e.g. marvin.cast.map()

inputs = [
    "I drove from New York to California.",
    "I took a flight from NYC to BOS."
]
outputs = marvin.extract.map(inputs, target=str, instructions="2-letter state codes")
assert outputs  == [
	["NY", "CA"], 
	["NY", "MA"]
]

[zzstoatzz]

woot woot! i've been wanting to back out map forever

functional marvin ftw 🎉