exact: Remove small_mul

All current cases can be handled by shift.
We can add this later if we need it.
exact: Replace GMP on small integers with custom generated code

gen-exact makes straight line routines to add, subtract, multiply,
square, and shift fixed size exact integers.  These routines rely
heavily on __uint128_t, and therefore turn on only for 64-bit gcc
compatible compilers.

This speeds up brd_to_svg.py in plan from 31.523 s to 23.028 s.