Convert methods to free functions

Convert methods to free functions

Making the GC an object makes no sense, since you can't instantiate it
more than once, it just make the code unnecessarily extra indented.

The GC struct now only have attributes (several renamed) and they are only
grouped for clarity (and to make easier to calculate the GC memory
overhead). All methods are converted to free functions that uses a global
instance of the GC struct.