big-cmp, vm/llvm support, fix karatsuba.
big-cmp, vm/llvm support, fix karatsuba.

big-cmp: new fun.
digits-add: simplified.
digits-lshift: check for empty digs.
mul2n1,mul2-half: use result vectors.
mul2: choose by %backend.
digits-mul: in school, make `a` bigger.
KARATSUBA-CUTOFF: measured.
karatsuba: final sum was incorrectly shifted.
divschool0, div2b1: use result vectors.

samrushing/irken-compiler,samrushing/irken-compiler,samrushing/irken-compiler,samrushing/irken-compiler