Move async_io.rs, fcntl.rs, and request.rs into block/src/io/. These
files are generic I/O infrastructure shared by all formats rather
than format specific code.
The io/ directory name clashes with std::io in lib.rs, so the module
is declared as io_impl via #[path] and the submodules are re-exported
at the crate root to keep existing import paths working.
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
