`externref` without allocations/hashmap, handled by the host application

[IndianBoy42]

Is your feature request related to a problem? Please describe.
I want to use externref to pass some handles to/from the wasm module, theres going to be very few and all statically known. Keeping a std::array<void*> is more efficienct and predictable (memory allocation) than the current map approach that externref_obj2ref and ref2obj use

Describe the solution you'd like
Either module_inst_t or exec_env_t could contain function pointers for the ref2obj and obj2ref, the host application could override them to replace the builtin functions

Describe alternatives you've considered
Just passing uintptr_t directly, no mapping or indices. basically not using externref at all or only on the module side (is this compatible?)