Difficulty

[robinmonjo]

Part of https://github.com/robinmonjo/coincoin/issues/9

This PR brings a lot of maturity in the coincoin blockchain !

• "real" difficulty setup and not just a "dumb" leading zeros check
• complete module to help people understand and pick an appropriate difficulty target
• docs on how to pick a difficulty according to the setup

Reviewers (if any 🙃) should focus on the doc to make sure it's clear and written in correct english. I learned a lot about blockchain difficulty, proof-of-work and how sha256 can be used to "control" the time in an untrusted network. This was really interesting so I'd like the doc to make it clear and interesting for readers.

PS: tables in the doc are hard to read in the diff, you can check theme here: https://github.com/robinmonjo/coincoin/blob/difficulty/apps/blockchain/docs/difficulty.md

PC2: I promise @yordis next PRs will focus on typespec (https://github.com/robinmonjo/coincoin/issues/14) + more tests (no feature)

[yordis]

Niiiiicee!

[robinmonjo]

@yordis handled your comment, feel free to approve the changes 🙃

[yordis]

@robinmonjo 😆