Adds cases for asynchronous functions with the proptest macros and a
test case using tokio.

Should work for both tokio::test and async_std::test attributes.

Support for async closures not added as they are not available
in the latest stable rust, but they are not supported
by the test attributes currently regardless.

Additionally as async/await syntax requires rust 1.39
sets minimum CI version to 1.39.

tempfile requires remove_dir_all which no longer supports rustc < 1.40.