regalloc2 0.5.1

Backtracking register allocator inspired from IonMonkey
Documentation
## regalloc2: another register allocator

This is a register allocator that started life as, and is about 50%
still, a port of IonMonkey's backtracking register allocator to
Rust. In many regards, it has been generalized, optimized, and
improved since the initial port, and now supports both SSA and non-SSA
use-cases. (However, non-SSA should be considered deprecated; we want to
move to SSA-only in the future, to enable some performance improvements.
See #4.)

In addition, it contains substantial amounts of testing infrastructure
(fuzzing harnesses and checkers) that does not exist in the original
IonMonkey allocator.

See the [design overview](doc/DESIGN.md) for (much!) more detail on
how the allocator works.

## License

This crate is licensed under the Apache 2.0 License with LLVM
Exception. This license text can be found in the file `LICENSE`.

Parts of the code are derived from regalloc.rs: in particular,
`src/checker.rs` and `src/domtree.rs`. This crate has the same license
as regalloc.rs, so the license on these files does not differ.