Make the OOM hook return `()` rather than `!`

Per discussion in https://github.com/rust-lang/rust/issues/51245#issuecomment-393651083

This allows more flexibility in what can be done with the API. This also
splits `rtabort!` into `dumb_print` happening in the default hook and
`abort_internal`, happening in the actual oom handler after calling the
hook. Registering an empty function thus makes the oom handler not print
anything but still abort.

Cc: @alexcrichton

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