implement printf() and friends

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It'd be good to have an actual printf(): http://en.cppreference.com/w/c/io/fprintf

But I do also want a more Python-esque format():

format("foo {} {} bar", 3, "meep"); //=> "foo 3 meep bar"

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This is in-progress on the printf branch.

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a lot of stuff from Ali should be pulled out into a library that's not building off an actual standard. format(...), if it ever is implemented, should fall into that category.

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#46 added a solid printf() implementation. I opened #48 about the functionality related to printing floating point numbers, since I feel that can be a separate thing.