Lexer truncates u64 literals outside u32 range

[jandem]

Consider this:

use std;

fn main() {
   let x : u64 = 2000000000u64 * 4u64;
   log_err x;

   let y : u64 = 8000000000u64;
   log_err y;
}

The output is:

rt: f462:main:main:                   8000000000
rt: f462:main:main:                   3705032704

It looks like the lexer uses 32-bit integers in scan_number etc.

Noticed this while trying to solve some Project Euler problems using Rust.

[boggle]

The basic analysis is correct but this is more involved, the use of int instead if u/i64 goes up via token::LIT_MACH_INT, ast::lit_mach_int all the way down to tycheck and trans. Will require some work fixing this. Secondary issue would be proper handling of signedness.