[MEDIUM] preapre st_vsprint for object printing

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
[MEDIUM] add sto_printf, sto_sprintf, sto_fprintf

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
[MEDIUM] finalized sto_printf variants

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
[MINOR] include st_object.h in .h that requires it

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
[MEDIUM] un-static sto_sscanf

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
[TEST] test sto_prinf, sto_scanf

Signed-off-by: etienne basset <etienne.basset@numericable.fr>