Support for tokio single-threaded runtime

[xfbs]

Hey! Love this crate. Was wondering if you think it made sense for me to add support for a tokio single-threaded runtime. I think that might be advantageous.

[frozenlib]

I have added support for specifying functions to execute asynchronous blocks in #[proptes(async = ...)]. (351a36f)

Using this feature, you can write a test using tokio's single-threaded runtime as follows

use std::future::Future;

use proptest::{prop_assert, test_runner::TestCaseError};
use test_strategy::proptest;

fn tokio_ct(future: impl Future<Output = Result<(), TestCaseError>>) -> Result<(), TestCaseError> {
    tokio::runtime::Builder::new_current_thread().build().unwrap().block_on(future)
}

#[proptest(async = tokio_ct)]
async fn async_expr() {
    prop_assert!(true);
    tokio::task::yield_now().await;
}