Support multiple compilers

Currently the system uses the default encoder.
We should add support for:

- [ ] singlepass
- [ ] cranelift
- [ ] llvm

```php
$instance = Wasm\InstanceBuilder::fromWat($wat)
    ->withCompiler(Wasm\Compiler::LLVM)
    ->build();
```

https://wasmerio.github.io/wasmer-python/api/wasmer/wasmer.html#engine
