mixed-width integer operations

[enricozb]

Fixes #109

Adds support for mixed-width integer operations.

• Operators can now be prefixed with one of u8, u16, u32, u60, i8, i16, i32. Omitting a prefix defaults to u60.
• Numbers are stored as i64, so when displayed they include the sign.
• Number literals can be prefixed with -.

[HigherOrderBot]

Perf run for a21cefb:

file              mode        main          a21cefb17bfd
========================================================
merge_sort        intr-singl       7.133 s       6.981 s
                  intr-multi       3.679 s       3.595 s
--------------------------------------------------------
boom              intr-singl       2.230 s       2.252 s
                  intr-multi       2.898 s       2.248 s
--------------------------------------------------------
bitonic_sort_lam  intr-singl      11.521 s      10.836 s
                  intr-multi       5.597 s       5.391 s
--------------------------------------------------------
radix_sort_lam    intr-singl      10.087 s       9.391 s
                  intr-multi       4.814 s       4.734 s
--------------------------------------------------------
sum_tree          intr-singl       9.318 s       9.133 s
                  intr-multi       4.365 s       4.425 s
--------------------------------------------------------
c2                intr-singl       0.000 s       0.000 s
                  intr-multi       0.001 s       0.001 s
--------------------------------------------------------
sum_tail          intr-singl       1.130 s       1.117 s
                  intr-multi       1.277 s       1.112 s
--------------------------------------------------------
sum_rec           intr-singl      14.492 s      13.384 s
                  intr-multi       7.338 s       7.020 s