Automatic handling for Wasm modules that export memory and tables #12

[shravanrn]

Currently RLBox only Wasm modules that import memory and tables (-Wl,--import-memory -Wl,--import-table) . With this PR, RLBox also supports Wasm modules that export memory and tables (-Wl,--export-all -Wl,--export-table). The handling is completely automatic (via metaprogramming) and rlbox_wasm2c does not have to be "told" whether a particular Wasm module transpiled with wasm2c is importing or exporting its tables.

Caveats

1. While exported memories are now supported, note that this will not be as efficient as Wasm modules that use imported memories created by RLBox --- this is because RLBox creates memories with alignment properties which make its internal operations (such as pointer swizzling) faster. Without this optimization, we have to fallback to simpler pointer swizzling.
2. rlbox_wasm2c assumes that the memory and table are both exported or both imported. We don't handle mixing and matching.

[squk]

Thank you!