Add Least Common Multiple

[jasonbobier]

This PR adds two least common multiple functions:

• lcm(T, T) -> T
  A simple version that will trap if T is not capable of representing the result.

• leastCommonMultiple(T, T) throws -> T
  A throwing version that instead of trapping, throws and error that contains the full width result if it cannot be represented in T.

Unlike other functions, such as gcd(), it is not uncommon for least common multiples to be unrepresentable in its fixed width integer. The lcm() function is offered in cases where this doesn't matter or where it is extremely unlikely to happen, such as BigNums. In other cases, the leastCommonMultiple() function is available. This will throw instead of trapping and the thrown error will contain the high and low words of the result saving the user from having to call leastCommonMultiple() again (and thus incurring the internal gcd() costs).

[jasonbobier]

This PR requires that apple#320 be resolved and merged and should not be merged until that is done.