Formulae for MMIX and CWEB

Here's a formula for MMIX tools. MMIX is a 64-bit RISC architecture designed by Donald Knuth. It is intended to be realistic and include a simple, elegant, easy-to-learn assembly language. The main audience are people reading TAOCP or learning assembly programming. (Is that too niche? I don't think so; TAOCP must be one of the most intended-to-be-read books for programmers!)

The mmix formula installs the following binaries:
* `mmixal`: assembler
* `mmix`: instruction level simulator
* `mmmix`: meta-simulator (don't ask me, I'm still learning about it all)
* `mmotype`: utility to display object format files

There's also a formula for CWEB, a literate programming system that MMIX is implemented in (so it's a build dependency).