`core_intrinsics` should be marked as an internal feature

[RalfJung]

Using the intrinsics feature triggers a warning about this being "internal to the compiler or standard library", but core_intrinsics does not. Clearly core_intrinsics is internal as well, so it should also get the warning.

core_intrinsics is a library feature though, not a language feature -- does the lint support that?

Cc @Nilstrieb

[RalfJung]

There are a lot of other library features that should probably also get the lint, namely all the ones with the _internals suffix.