Allow for IO abstraction left on library consumer

[erin-desu]

Use Case:

Sled, as far as I know currently uses OS-provided I/O and FS facilities. This may be an issue on platforms, like in embedded cases where there may not be any conventional filesystem or system-provided I/O facilities.

Proposed Change:

Introduce a trait for providing custom I/O implementation.

Who Benefits From The Change(s)?

Embedded and other platforms not complying fully with Rust standard library.

Alternative Approaches

Nothing on this project's side, the library user can technically implement their own stdlib (subset).