Add (optional) early collection support

Add (optional) early collection support

When early collection is enabled, the collection will be triggered before
the memory is actually exhausted (the min_free option is used to determine
how early the collection should be triggered). This could remove a little
pressure from the GC when eager allocation is not available (or when eager
allocation makes the heap grow too much).

The option is disabled by default and can be disabled with the
early_collect option.