Use cargo expand as a library

[fzyzcjy]

Hi thanks for the library! When developing https://github.com/fzyzcjy/flutter_rust_bridge, I find it great if cargo-expand could be used as a Rust function, such that there is no need to separately install a binary, etc.

[dtolnay]

I would prefer not to build a Rust API into this crate, but someone else should feel free to maintain a library for this.

[fzyzcjy]

Thanks for the reply