Compiler devnotes: Machine specs

I'll be writing a few specs to validate my design, but spending most of my time on other parts of the toolchain. Adding new targets is straightforward to make it easier for anyone to write an architecture, so I don't feel the need to personally write a whole bunch of them unless I'm bored some weekend.

Edit:

>Mill uses two program counters, and every wide instruction is split into two parts. One of the program counters counts backward. So, the code of every linear instruction block is executed from its middle to outside by two almost independent decoders.

Oh dear lord.