These functions are unused and never exposed publically - along with it,
get rid of `GC_Strategy` - although it's possible someone could use this
`enum` for their own code it seems unlikely.
The forward declarations cause `Exception` to be inferred - also,
`llrecv` is an internal implementation detail and the type of the
received item is controlled by generics, thus the ValueError raised
there seems out of place for the generic api.
