* Field: Default
It's done for primitive types like `u64`, so seems conventional, and some code in mir-core expects it.
* HashOut::ZERO
* Default for HashOut
* fmt
* pub elements
* Debug
* rand_from_rng
- Made some methods public, if they seemed like they'd be useful crates that depend on plonky2, and seemed like good/stable APIs
- Deleted a few things I didn't think seemed very useful
- Left a few for now that I was on the fence about
It's just a wrapper around `Target`, which signifies that the wrapped `Target` has already been range checked. Should make it easier to audit code that expects bools.
No functional changes here. The biggest change was moving certain files into new directories like `plonk` and `iop` (for things like `Challenger` that could be used in STARKs or other IOPs). I also split a few files, renames, etc, but again nothing functional, so I don't think a careful review is necessary (just a sanity check).
