detects redundant imports that can be eliminated.
for #117772 :
In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.
Stabilize the `core_c_str` and `alloc_c_string` feature gates.
Change `std::ffi` to re-export these types rather than creating type
aliases, since they now have matching stability.
It looks like the last time had left some remaining cfg's -- which made me think
that the stage0 bump was actually successful. This brings us to a released 1.62
beta though.
