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).