Remove async-convert

[ifsheldon]

As in my commit to the downstream fork async-openai-wasm, it's possible to remove async-convert crate. Without async-convert, we don't need to use async-trait either, which remove extra boxing. This also shrinks binary size. I would be happy to upstream this change in a PR.

[64bit]

Thanks for suggestion, PR is welcome!

Also MSRV for this crate will need to change to 1.75? https://blog.rust-lang.org/2023/12/28/Rust-1.75.0.html#whats-in-1750-stable

I don't follow why it shrinks binary size?

[ifsheldon]

Also MSRV for this crate will need to change to 1.75?

Yes

I don't follow why it shrinks binary size?

This is my guess with some ground. If you cargo-expand to see the result of an impl block by #[async_trait::async_trait], you'll see some complicated code that is just for boxing, not for the application logic. By removing these auxiliary code, the binary size should be smaller.