Start work adding M0+

This isn't finished (not included in Rakefile and not used anywhere), but hoping to "dibs" it by throwing a pull request in. I've been working on my own rust/cortex library, but after this came out I figured I'd jump ship.

I've not included the multitasking files (sched.*) as I wasn't able to find the best way to push regs r4-r11 to the stack. The M0+ only allows r4-r7 for an STM/LDM/PUSH/POP and STMDB isn't available.