Fix a bug arising from putting generation 1 objects into the

checkobj array (via the mut macro that expands to gc_mutated).
The garbage collector assumes that checkobj has only generation 0
objects, which all exist in the freshobj array, which is subject
to a sweep. So gen 1 objects in checkobj are never cleaned up
properly: they do not have their REACHABLE flag reset, or
their generation restored to 1. To fix this, a new array for these
objects is introduced separate from checkobj.

* gc.c (MUTOBJ_VEC_SIZE): New preprocessor symbol.
(mutobj, mutobj_idx): New static array and integer.
(mark_obj): Check for REACHABLE flag before checking the full_gc
flag and generation, since those cost additional memory accesses.
(mark): Mark the objects in the new mutobj array.
(sweep): Sweep the objects in the mutobj array.
(gc): Reset mutobx_idx to zero after gc.
(gc_set): Rearrange logic. In the case that the checkobj array
is full and a gc is done to make room, there is no point in
adding to the array: the gc pass moves all babies to generation 1,
so the object that was passed into the function is no longer a baby.
(gc_mutated): Rewrite in terms of mutobj rather than checkobj,
fixing the bug.

* HACKING: Improved documentation of GC. Describe mut macro
and mutobj array.

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