Add cista::*::cstring type

This new type is able to store a trailing \0 character, without
compromising one byte for storage when the string is a small-string.

Storage of NUL character within data is also supported.

This is inspired by
felixguendling#187 (comment).

See felixguendling#187.

Author: khng300

include/cista/containers.h

@@ -3,6 +3,7 @@
 #include "cista/containers/array.h"
 #include "cista/containers/bitset.h"
 #include "cista/containers/bitvec.h"
+#include "cista/containers/cstring.h"
 #include "cista/containers/fws_multimap.h"
 #include "cista/containers/hash_map.h"
 #include "cista/containers/hash_set.h"