Cleanup EC algorithms

[fcelda]

Some changes I propose and would like to review before merging.

• I've unified the algorithm parameters for consistency. So it's always a bullet with:
  name - description with no other definitions
• I've unified the steps in the algorithm so that we don't mix normal sentences with pseudocode.
• I've added "INVALID" variant into description of algorithms' output where the function could fail.
• I rewrote the repeat-until loop into a while loop in hash_to_curve for better readability.
• I've cleaned up the EC ciphersuites definitions. The 2n and cofactor parameters are within the bullet specifying curve. The m parameter is determined by EC2OSP implementation, so I moved it into this bullet.
• EC2OSP for ED25519 has m=32 not 33.

[goldbe]

Jan for my cref:
need to confirm that os2ecp outputs INVALID (or something like that) if conversion fails!!

Did you actually confirm this?