r_string_to_number should return R_FAIULRE when error occurs
Removed dead code
Added gc_enabled, gc_disabled & gc_enabled_p

By default, GC is disabled, and is enabled after RState initialization.
Avoided to free NULL pointers during vector_finalize and bytevector_finalize
Bugfix: added GC mark procedure for the reader
Added GC mark function for primitive procedure object
Added more assertions for pair functions
Removed several unused API functions
Added several arithmetic primitive procedures
Removed the unused field last_error from struct Reader
White space formatting and string format adjusts