BigInt support

[WillDaSilva]

Currently if you create a large enough number they lose precision. Larger still and they become inf. It'd be nice to have a type for arbitrarily large integers.

Some similar features that would be separate from this issue, but also worth considering, include:

• fixed width int types (e.g. Int64, UInt64, Int16, etc.)
• complex number type
• arbitrarily large/precise real number type
• rational number type