Begin work on new memory layout for all objects and GC.

The layout should actually be very similar to the old layout, but
the code will be much easier to change and should be more portable.

src/core/debug.c

@@ -54,7 +54,7 @@ void janet_debug_find(
     JanetFuncDef **def_out, int32_t *pc_out,
     const uint8_t *source, int32_t offset) {
     /* Scan the heap for right func def */
-    JanetGCMemoryHeader *current = janet_vm_blocks;
+    JanetGCObject *current = janet_vm_blocks;
     /* Keep track of the best source mapping we have seen so far */
     int32_t besti = -1;
     int32_t best_range = INT32_MAX;