Revise struct declarations; modify Actor updates.

Add:
	src/termcrate.h
		-add a Geometry struct, which contains values common to Actor
		structs (Bullet, Enemy); greater readability, and makes
		collision() function easier because it intakes the same inputs
		(two Geometry structs) instead of actor-specific structs.

	src/termcrate.c
		-add collision() function, to return whether two Geometry
		structs have "collided". Add necessary calls to updateEnemies()
		and updateBullets().

Mod:
	src/termcrate.h
		-compound Enemy and Bullet structs into a single Actor structs,
		as they're effectively identical; will revise later, if necessary.
		-add typedef declarations to structs, which increases
		readability.