Add `_timeout` variants to blocking executions

[wyfo]

A common pattern for blocking functions in Rust is to provide a _timeout variant, see rust-lang/rust#46316. However, this crate currently doesn't provide a way to interrupt blocking executions, which prevents it to be used in some workflows.

Here is the context of my main use case:
I've implemented a channel, and because it's a channel, the synchronization is naturally implemented using asynchronous code and primitives like AtomicWaker. However, I also needed to expose a synchronous API, with timeout/deadline capabilities. Because futures_executor::block_on didn't have a _timeout counterpart, I couldn't rely on it, and had to write the synchronous API (using thread::park and reimplementing AtomicWaker) myself.
This feature request would simplify things a lot, allowing me to fully delegate the synchronous implementation to the future_executor crate. (Performance may be a little bit worse, because of mandatory TLS access even if waker is not used, but the simplification should be worth it)