Change JSObject.weak_ref_list to use a bit and external hash table.

1. Add a generic hashmap implementation.
2. Change Error `JSObject.is_uncatchable_error` flag to `JSObject.u.error` struct, leaving room for weak ref flag bit.
3. Compat `JSString.hash_next` to 31-bits (can be safely compat to 28-bits), leaving room for weak ref flag bit.

Save memory for each JSObject (8 bytes on 64-bit platform, 4 bytes on 32-bit platform).

TODO

@@ -47,7 +47,6 @@ Optimization ideas:
 - add implicit numeric strings for Uint32 numbers?
 - optimize `s += a + b`, `s += a.b` and similar simple expressions
 - ensure string canonical representation and optimise comparisons and hashes?
-- remove JSObject.first_weak_ref, use bit+context based hashed array for weak references
 - property access optimization on the global object, functions,
   prototypes and special non extensible objects.
 - create object literals with the correct length by backpatching length argument