[stdlib] Add more constants, and tweak descriptions.

[helehex]

Not sure if it's better to just use the nan[DType]() function, but I find it convenient to have a FloatLiteral one.

If anyone wants me to add the twelfth root of 2, I am more than happy todo so.

[helehex]

It's probably smart to have tau = 2 * pi instead of writing it all out, to at least ensure consistency between them.. but I'm not sure if it matters in practice. Let me know I guess.

[soraros]

utils.numerics.nan etc. are also exported through math.

[helehex]

oh yeah, I was going to change the sub-module docstring to something like:
"""Defines mathematical constants, accurate to 64 decimal digits."""

[helehex]

Don't worry @soraros, I have a feeling it will all work out in the end :]

[helehex]

Also, pi*2 == tau is still True, so that's always good.

[helehex]

I just remembered, you can also do things like Float64.MAX to get inf. Maybe there's too many ways to do a simple thing.