Vector class improvements

Nicer string representation.
Caching of length once it's been calculated once, to avoid
repeated square root calculations.
Made length, squared_length, unit properties for more
intuitive use.
Removed the code in calculate_collision_direction()

It wasn't working, and I came up with a better solution that I'll try
when I get around to it.
Changed approach_target_speed(), renamed to accelerate()

Renamed the approach_target_speed() method. New name is accelerate().
It now uses vectors instead of tuples, and does its thing in
significantly fewer lines of code.
Also changed actors' acceleration property from a vector to a single
number.
Apparently changed animbuilder?

Fucked if I know. Different line breaks?
Added collision effects to hero

Trip, tumble, rise, knockback now work roughly as intended.
Updated .gitignore

Added filenames ending in ~. Backups are nice, but git already does
that.
Visible hitboxes

Added some ugly, ugly code to show the hitboxes of GameObjects.
Will put this in a method and make it debug-only later.
Knockback tweaks