Dedicated opcode for % and // (code size and instruction cost)

From Motoko's #153 review. Each % compiles to about 11.7 KB and each // to about 22.7 KB (about 1.2 KB before #149), so a program with about 450 of them exceeds the 10 MiB load cap. 200 × (% 1e300 (+ i 0.5)) hits the 64 MiB memory limit. Small int ops cost about 3.2x per op. Consider a dedicated opcode, with parity tests on all three paths.