Ben Joffe's Mersenne weekday trick vs plain %7 in clang, V8, and CPython on M4

Benchmarked on an M4 Mac mini: Ben Joffe's 2-instruction weekday hack beats plain %7 by 1.6-6.4x in clang, Rust, and V8, loses 3x in CPython. Plus a 25x V8 -0 deopt trap.