Now that WAMR supports multiple memory instances, this PR adds some APIs
to access them in a standard way.
This involves moving some existing utility functions out from the
`WASM_ENABLE_MULTI_MODULE` blocks they were nested in, but multi-memory
and multi-module seem independent as far as I can tell so I assume that's okay.
APIs added:
```C
wasm_runtime_lookup_memory
wasm_runtime_get_default_memory
wasm_runtime_get_memory
wasm_memory_get_cur_page_count
wasm_memory_get_max_page_count
wasm_memory_get_bytes_per_page
wasm_memory_get_shared
wasm_memory_get_base_address
wasm_memory_enlarge
```
926f662231