refactor: MoonBit support update

[peter-jerry-ye]

This PR ships a few updates to keep up with a few MoonBit changes to eliminate the warnings as much as possible.

• The borrow / own attributes are added for the ffi functions

This PR also refactored the codegen part of MoonBit to make it more modular and more readable, hopefully.

• The experimental async support code are extracted for better readability
• Some redundant logic (such as cleanup list, code gen fragments, etc) are simplified
• The inline wasm code snippets are generated where they are used, so there's no longer necessary for a separate package.

I expect to refactor the async support in the next PR.

[alexcrichton]

Thanks!