Async support using tokio for Read/Write calls

[SapryWenInera]

I think using async for the parts which use read or write calls could improve performance and allow tokio's rt-multi-thread to use multiple cores on write/read of a file.

[Pr0methean]

See #72 and #73 for the async and parallelization efforts that have been made so far.

[SapryWenInera]

I'm considering giving it a try if is it ok with you.

[Pr0methean]

Sure! The only other major work I know of going on right now is #93, which probably won't overlap much.