Add LeastCommonMultiple and GreatestCommonDivisor to math package

[SeanTAllen]

Adds code for determining least common multiple and greatest common divisor to the standard library math package.

[SeanTAllen]

@ponylang/committer this is ready for another review. I've address @jemc's comments.

[SeanTAllen]

@EpicEric are you good with this now?

[EpicEric]

I'd expect some tests for the new functionality. Other than that, I left a minor style suggestion.

[SeanTAllen]

What do you expect as tests? I don't see much value in a couple of unit tests. I could see some value in property tests if ponycheck becomes part of the standard library.

If you have an idea for tests, please feel free to add.

[EpicEric]

I'd also prefer property tests, but since these aren't in the stdlib yet, I personally feel that simple tests are better than no tests.

Some simple tests could be:

1. Prime arguments (GCD is 1, LCM is the product)

t.assert_eq[U32](GreatestCommonDivisor[U32](373, 1231)?, U32(1))
t.assert_eq[U32](LeastCommonMultiple[U32](373, 1231)?, U32(373) * U32(1231))

2. Equal arguments (GCD and LCM should both be equal to the argument)

t.assert_eq[U32](GreatestCommonDivisor[U32](0x506F6E79, 0x506F6E79)?, U32(0x506F6E79))
t.assert_eq[U32](LeastCommonMultiple[U32](0x506F6E79, 0x506F6E79)?, U32(0x506F6E79))

[SeanTAllen]

@EpicEric tests added