Impedance: Tools to mix blocking and async code

cargo +nightly bench:

``` $ cargo +nightly bench Finished bench [optimized] target(s) in 0.62s Running target/release/deps/impedance-6efcafb7e2533f63

running 2 tests test tests::itworks ... ignored test tests::itworks_panic ... ignored

test result: ok. 0 passed; 0 failed; 2 ignored; 0 measured; 0 filtered out; finished in 0.00s

 Running target/release/deps/spawn-f5f0f45a25148c29

running 4 tests test noadaptivefast ... bench: 2,936 ns/iter (+/- 222) test noadaptiveslow ... bench: 112,729,812 ns/iter (+/- 5,704,146) test withadaptivefast ... bench: 4,605 ns/iter (+/- 1,575) test withadaptiveslow ... bench: 11,895,813 ns/iter (+/- 1,509,361)

test result: ok. 0 passed; 0 failed; 0 ignored; 4 measured; 0 filtered out; finished in 45.16s ```