Add a Mark-and-Sweep GC to Your VM

Add a mark-and-sweep garbage collector to your VM in C.

Requirements:

• Define an Obj struct with type, mark bit, size, next pointer (for allocation list), and a union for data (strings, closures, or lists)
• Implement allocate_object() that allocates from a fixed-size heap or via malloc, adding the object to an allocation list
• Implement mark_object() that sets an object's mark bit and recursively marks any objects it references
• Implement mark_roots() that marks all objects reachable from the operand stack, all call frame locals, and global variables
• Implement sweep() that iterates the allocation list, frees unmarked objects, and clears marks on live objects
• Implement collect_garbage() that runs mark_roots() then sweep(), printing heap stats
• Demonstrate allocating several objects, letting some become unreachable, and running GC
• Show heap stats: total objects, bytes allocated, bytes freed, bytes remaining

Test:

• Show GC running
• Show mark phase identifying live objects
• Show sweep phase freeing dead objects
• Show heap statistics before and after collection