Support U64

[porcuquine]

This PR adds support for fixed-width (64-bit) unsigned integers. With an eye to potential future work, it defines a general UInt enum, of which UInt::U64 is the only initial variant.

Arithmetic behaves as though mod 2^64. Division is 'integer division'.

Literal U64s are denoted with a suffix, either u64 or U64 directly following a valid number. It is a reader/parser error to specify a U64 from a number greater than the maximum U64.

The num operator coerces U64 to Num, with no loss of precision.

The u64 operator coerces Num to U64, preserving only the least-significant 8 bytes.

Arithmetic operations that mix U64 and Num yield Nums and are performed by first coercing the U64 to Num.

The u64 suffix cannot be combined with negative or fractional notation.