Fixed a comment to SSTRING() macro.
Removed old unnecassary comment to s_free.
Made empty_sstring() not free memory and changed documentation to reflect that.
Removed s_sstonews().

They are no longer necessary as the string inside them should always be a C string.
Changed the name of s_free() to be up to date.
Added a new version of empty_sstring and started testing it.

Just testing if lone restrict keyword in argument list makes any difference in optimization of the code.