Add while loops, GC mark-and-sweep, and malloc block reuse

- Lexer: add INDENT/DEDENT tokens, <, >, : operators, while keyword,
  closing DEDENT emission, include guards
- Parser: add NODE_WHILE with while_loop union, parse while/cond/body
  blocks, include guards
- Eval: add while loop evaluation, GC integration with roots from env,
  debug and gc flags, <, > comparison operators
- GC: implement mark-and-sweep collector with 3 stages (mark roots,
  sweep unmarked, join free blocks)
- Allocator: block reuse via first-fit search with splitting, exponential
  heap growth, NULL check on malloc, include guards, marked field in metadata
- Object: add include guards, fix include to use allocator.h

projects/while.j

@@ -0,0 +1,4 @@
+x = 0
+while x < 100000000:
+    x = x + 1
+print x