Add const to strings in skeletons/

This reverts commit 233be538.

Add (void) when st is unused

Missing void in _new_stack()

Fix duplicate 'const' warnings

Fix issue 144

Merge pull request #154 from brchiu/vlm_master_merge_pr99_mouse_2c8d366b_solve_prob_for_s1ap_nbap_dsrc

Merge PR99 and its fixes to support parsing Information Object and Information Object Set

Since the typedef of asn_INTEGER_specifics_t also contains a const the
compiler complained about:
"warning: duplicate ‘const’ [-Wpedantic]"

This removes the GCC warning:
"warning: function declaration isn’t a prototype [-Wstrict-prototypes]"