Instead of assuming memory index 0,
look up an export with the name "memory".

to be consistent with cconv_deref_func_ptr

to prepare for other strategies to pick a memory to use.

there are no foreseeable users. let's simplify.

maybe it's a bit clear to do:

```
struct host_instance_using_cconv {
    struct host_instance hi;
    struct meminst *mem;
    struct tableinst *func_table;
}
```

but i didn't bother to do that right now. i might revisit later.

also, do the same for the table as well.

also, make the repl initialize them accordingly.
(only for memory, just because wasi doesn't happen to
use function pointers.)

dyld_func_table is currently unused.
but i added it for consistency.
it would be necessary if an embedder provides host functions
which use function pointers.

i'm not entirely happy with the use of a callback.
i may revisit this later.

cf. WebAssembly/wasi-testsuite#100