Add weak references in the form of weak tables.

Any references exclusively held by a weak table may be collected
without the programmer needing to free references manually. A table
can be setup to have weak keys, weak values, or both.

src/core/state.h

@@ -121,6 +121,7 @@ struct JanetVM {
 
     /* Garbage collection */
     void *blocks;
+    void *weak_blocks;
     size_t gc_interval;
     size_t next_collection;
     size_t block_count;