More variables (plus a couple of enhancements)

This patch adds half a dozen new variables related to the player HUD:

* **healthpoints** — the player's current health, in half-hearts
* **armorpoints** — the player's current total armor, in half-… er, armors (icons)
* **foodpoints** — the player's current food level, in half-shanks
* **foodsaturation** — the player's current saturation level, an otherwise invisible value
* **airticks** — the player's remaining air supply, in game ticks
* **underwater** — `true` if the player's head is in liquid (i.e. if they're holding their breath), otherwise `false`

A few "enum-style" variables (`difficulty`, `gamemode`, and `dimension`) have been changed to use names supplied by the game.  This allows localized difficulty and gamemode names as well as the names of mod dimensions.  Additionally, `*id` variants have been added for those three, plus `biome`, which return the variable as a number, so that logic can be performed using them.

Finally, the `equipped*` variables have been folded into the armor block, as the handling was virtually identical.