Read write helpers

[faern]

Motivation

tokio-fs exposes a similar interface as std::fs. Some time ago very convenient helpers for reading and writing entire files with a single function call were added to the standard library. The stabilization discussion is available here: rust-lang/rust#46588

tokio-fs don't have these convenient helpers. So just opening a file and reading/writing to it is slightly more cumbersome than it would need to be. I figured that if the community reached consensus on adding these to the standard library, the same arguments would hold for tokio-fs as well.

Solution

Adding two custom futures, that join File::open + read_to_end and File::create + write_all.

The read future also polls the fs for metadata and pre-allocates the buffer accordingly. Just like what std::fs::read does.

[carllerche]

👍 Looks great to me ❤️