[stdlib] Begin prototyping new Integers
[stdlib] Begin prototyping new Integers

We're starting with some low-level bit-bags that essentially expose
LLVM's integers.  They will have both signed and unsigned operations.

On top of these, we can build a LowLevelBigInt, so that the widest
fixed-sized low-level Int can be sign/zero-extended.  Then we can build
higher-level Int and UInt types on these, including (signed) BigInt.

Swift SVN r31693

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