Beautifying the rect template thing

It's much more readable now, and more intuitive as to what is actually
happening.
Fully fixing rect_intersection

It's actually correct now!
Fixing the UI_WorldView to work with the correct rect implementation
Minor main.c changes

Removing unnecessary #include

Changing main() return value to 0
Making value_in_range a macro

So it should work with any numeral type
Using self-made CONCAT macro instead of __CONCAT

I don't think __CONCAT is guaranteed to be defined by stdint.h so this
is just the better way to do it.
Removing rect_create

Why would I ever want a rect on the heap anyway, unless it's as part of
a larger struct? I wouldn't. This means I don't need to include stdlib.h
anymore, too.