Compute correct hit points for a given character using their chosen class levels, ability scores, with optional support for toughness or other modifiers.

[aaclayton]

Originally in GitLab by @gatesvp

Now that we have explicit class levels, we should be able to directly calculate a PCs HP from that data. This would match up with Adventurers League.

Notes:

• For multiclassing, we clearly need to know which class was "first". Though we need this if we ever want to track proficiencies as well, so this is reasonable to have.
• Not clear how we hook up modifiers from things like Dwarves or Feats like Tough that grant extra HP at every level.
• We likely want a manual override here, though we may be a little tight on space for that.

[aaclayton]

Re-scoping to %"D&D5E 1.3.1 - Continued Work on Level-Up"

[aaclayton]

marked this issue as related to #866

[aaclayton]

Doable suggestion, keeping this open if anyone wants to pick it up as a MR.