This makes io_uring the default in CLI, but makes it non-default in
core. Before, if one built CLI without io_uring, core still built with
it as it was a default feature. To accommodate for the change, all
bindings have been updated to select the feature, except for WASM which
has a separate fs implementation.
This also adds some #[cfg] and #[allow] to silence unused-* warnings,
which I discovered when testing with different features disabled.
Closes#942