Add prim_minus
Expand memory size to 8KB
Add sample of 5! computation
Replace prim_minus with prim_negate
Add sample of 5! computation using minus-1 instead of negate
Fix bungles
Add gc calls
Toggle DEBUG_GC
Revise alloc() to allocate all the objects in fixed 12 bytes
Add prim_car and verbose 5! computation sample
Support: \'-\'
Remove hard-coded object size
Rivise alloc() to allocate aligned object
Add code reading memo
Format code reading memo
Refine env structure
Add descriptions about `root' and change format to markdown
Commit temporarily
Rewrite gc() with simple mark&sweep GC
Merge remote-tracking branch 'origin/mark_and_sweep-1' into develop