API and code improvements

[jsign]

This PR contains multiple changes, such as:

• Document all exported methods in important packages so clients can rely on them.
• Refactor/Renaming of methods and constants with (IMHO) better alternatives.
• Fixed a performance TODO in MultiExp.
• Improved CI.
• Improved README, giving particular advice on how clients should use this repository.

I'll be creating a go-verkle PR to feel how this looks like from a client-side and get some feedback. So, leaving this as a draft for a bit for now.

Also, I've already updated a bit the repo top (description, web, tags):

I'm planning to take care of all panics(...) in a further PR.

[kevaundray]

This PR is amazing