Add mruby constants for the Z start values.

This makes it easier to modify a sprite’s Z position from a script.
Add array type ARRAY_LAVA.
Add lava sprite.

Uses bonusbox graphics, currently.
Use Crabmaster’s Lava images instead of the bonus box one.
Fix Lava collision rectangle
Make Mousecopy realize Lava
Disallow creating climbable lava and such nonsense
Make lava scriptable
Merge branch 'lava' into devel. Closes #32.