Restore some and remove some declarations for GC API in julia.h

[OndrejSlamecka]

Access to statistics was removed but the implementation is still present and useful. jl_gc_use was removed but declaration remained.

[imciner2]

Are you needing to use these in an embedded case (e.g. linking against libjulia)? If so, you also need to expose them in jl_exported_funcs.inc so that the proper mapping in libjulia is made. I am not sure if these internals should actually be exposed though, so I leave that decision to others that have more knowledge of the GC system.

[OndrejSlamecka]

Yes, I use these to monitor allocations in a setting with embedded Julia.

Whether these should be exposed is a fair question, however, they were exposed previously, and in fact the exports are already present (or removed in the removal case):

~/p/julia (gc-api) grep "jl_gc_get_total_bytes\|jl_gc_total_hrtime\|jl_gc_diff_total_bytes" src/jl_exported_funcs.inc
    XX(jl_gc_diff_total_bytes) \
    XX(jl_gc_get_total_bytes) \
    XX(jl_gc_total_hrtime) \
~/p/julia (gc-api) grep "jl_gc_use" src/jl_exported_funcs.inc || echo empty
empty