Add arbitrary-precision integer to std

A few notes on the implementation:

 - Any unsigned power of two integer type less than 64 bits in size is supported
 as a Limb type.
 - The algorithms used are kept simple for the moment. More complicated
 algorithms are generally only more useful as integer sizes increase a
 lot and I don't expect our current usage to be used for this purpose
 just yet.
 - All branches (practically) have been covered by tests.

See https://github.com/tiehuis/zig-bn/tree/986a2b3243d0454b8430a6adf4ad48611850c1b8/bench
for rough performance comparison numbers.

Closes #364.

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