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.
* allow defects to be caught even for --exceptions:goto (WIP)
* implemented the new --panics:on|off switch; refs https://github.com/nim-lang/RFCs/issues/180
* new implementation for integer overflow checking
* produce a warning if a user-defined exception type inherits from Exception directly
* applied Timothee's suggestions; improved the documentation and replace the term 'checked runtime check' by 'panic'
* fixes#13627
* don't inherit from Exception directly
