Feature request: poll_lock(cx) for Mutex

[black-binary]

I want to lock a Mutex in my poll_read method but it seems like that there is no way to lock and wait for a Mutex in a non-async function. My workaround is creating an async closure and getting it boxed and pinned, storing it somewhere else, and then polling it, which is really annoying and inefficient.

https://github.com/black-binary/ap-kcp/blob/6bf035ea0349165094bf44e13ff4b09cb2bf66ab/src/async_kcp.rs#L54-L67

I think it would be nice if there are some methods like poll_lock(cx).

I'd accept a PR for this method :)

[notgull]

I think a better way of doing this would be like in smol-rs/async-channel#33, where we have a manual Future implementation for the locking functions. This is basically what the PR (#5) does anyways, and we could probably make it more generic.