Create Codephrase struct and newtypes for it

[emilyalbini]

Before this PR, codephrases were represented in the codebase as Strings, and freestanding functions were used to generate and parse them. This was slightly unwieldy to work with, and for the integration in permslip I wanted to have stronger typing.

This PR replaces all of that with a new Codephrase struct, which wraps the raw U256. Converting from and to the codephrase representation now happens at ser/de time transparently.

To increase type safety, and to clearly delineate which codephrases are truncated and which are full, instead of using Codephrase across the codebase this also adds a macro to create newtypes of it, and creates a newtype for each use of it.

The only functional change compared to the previous implementation is how truncated codephrases are actually truncated. Before, we were always including 8 words, regardless of how many bits of randomness they represent, while the new implementation truncates the randomness at 88 bits. In practice, this results in 8 words 99.95% of the time and 7 words 0.05% of the time (never 9 words). I don't think this is a problem, as 88 bits was the randomness we were targeting anyway.

This PR is best reviewed commit-by-commit.

[plotnick]

This is fantastic work, thank you! I ❤️ the newtypes especially; I was just starting to worry about them, and now you've removed that worry.

I took the liberty of pushing two small commits that were so nitpicky I didn't even want to bother you with them, so I had the 🤖 do it instead. Please feel free to adjust as needed. I need to write a tiny house style guide so that at least my crazy is written down somewhere.