Add more number types

[oxinabox]

Following up on #191
It would be useful to have a few more number types.
In particular it would be useful to have Rational numbers and 128 bit integers.
Calculating grid-coefficients for higher-order finite differencingusing Rational{Int128} for intermidiate values before converting to Float64 at the end gained me several orders of magnitude in accuracy;
and presumably has other applications.

And I know people working in astronomy like there rediculously high precision floats because of working at different scales.

I suggest:

• Rational numbers
• Int128
• BigInt (GMP equiv)
• BigFloat (MPFR equiv)

[dougalm]

Good idea. There's a question of whether to do these in the compiler or at the library level. Where there's no special hardware support I tend to lean towards doing things at the library level. What does Julia do?

[oxinabox]

All* types in Julia are at the library level.
Including Float64 etc..
Julia provides the primative keyword to create words-of-bits,
which one then might use llvmcall (or ccall) to define the fundermental operations (that will in many cases have hardware support).
Some exceptions like Int64 are not literally defined at the library level because of bootstrapping reasons.

The types I mentioned above all in the julia standard library.
In packages there are a bunch more like ArbNumerics.jl and BitIntegers.jl and BFloat16.jl

_{*not technically true but close enough. One can get into semantic arguments easily enough.}