fs: add File::into_std and File::try_into_std methods (#1856)
fs: add File::into_std and File::try_into_std methods (#1856)

In version 0.1 there was File::into_std method that destructured
tokio_fs::File into std::fs:File. That method was lacking in
version 0.2.

Fixes: #1852
tokio-rs/tokio,tokio-rs/tokio