gc: Fix memory leak, improve sweep loop

A memory leak related to deferred finalization was fixed. The sweep loop has
been rewritten: It is now more readable and propably more efficient. An
outdated comment was updated.
mkx: Improve sweep loop of string table

The sweep loop of the string table has been rewritten: It is now more readable
and propably more efficient.
mkx: Add array of boolean
mkx: Add test for deferred finalization
mkx: Fix initialization of Core_ArrayList

The elements of the array internally allocated by Core_ArrayList are now properly initialized.