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
As discussed in
https://github.com/rust-lang/rust/issues/49668#issuecomment-384893456
and subsequent, there are use-cases where the OOM handler needs to know
the size of the allocation that failed. The alignment might also be a
cause for allocation failure, so providing it as well can be useful.
