Create proper abstraction so that we can use `WasmEngine` with multiple VMs

[Alex Beregszaszi]

In https://github.com/ewasm/hera/blob/master/src/hera.cpp#L297 should be able to do something like:

WasmEngine engine;
switch (engine) {
  case wasm_engine::binaryen:
    engine = BinaryenEngine{};
    break;
  case wasm_engine::wabt:
    engine = WabtEngine{};
    break;
}
engine.execute(...);