Helpers cleanup

[JustinDrake]

Addresses #918. It includes:

1. Grouping helpers by category
2. Renamings (for consistency, length, clarity)
3. Add top-level function comments where missing, and make them consistent
4. Remove neglected and sometimes misleading terminology section (to be readded post-freeze)
5. Various other cosmetic cleanups

[CarlBeek]

This is a much more sensible ordering of helpers, I really like it! I do have a few comments though:

• For consistency's sake, I think the Terminology section should be removed from 1_custody.md too.
• I am not a fan of the function naming used in the Misc section of 0_beacon_chain.md. Based on the name alone, I don't think I would be able to determine what a function does in many of the cases. See my individual comments for details. If you are averse to the use of get in function names in this section, what about using compute or calculate instead (this is already done in compute_committee).

[JustinDrake]

I tried making BLS_WITHDRAWAL_PREFIX a Bytes1 but broke something. (I think the spec builder is confused somehow.)

[djrtwo]

I think we should leave as is instead of the "default" value here. It is a configurable constant that might be changed in the future (or in different deployments)

[djrtwo]

I'd prefer to revert to my last commit and get this merged. We have other things to handle that are waiting on this PR

[JustinDrake]

I think we should leave as is instead of the "default" value here.

That's fine :) (Tried moving it to see if spec builder would be happier.) Making it Bytes1() is still probably the way forward.