refactor(moonbit): use internal abi

[peter-jerry-ye]

This PR made a lot of hand written inline wasm to call the builtin functions, so that:

1. less bugs
2. prepare for migrating the memory layout.

This PR also adapts the new test framework for MoonBit.

[peter-jerry-ye]

@alexcrichton The new test framework was great. It helped me find a number of bugs.

I wonder though if we can exclude MoonBit from the default language testing set for now? (same reason as why it's not in the CI, it's not that stable yet) I tried not to add MoonBit to ALL but then the codegen test won't run for MoonBit even if I set --languages moonbit.

[alexcrichton]

Thanks for this!

I'll work on integrating a better set of defaults into wit-bindgen test to get tests working for this with explicit opt-in, but for now seems reasonable to have this as-is

[alexcrichton]

Ok in #1246 I'm going to just require that --language is always passed to request something explicitly and avoid the "include everything" behavior.