add string functions, cleanup vector docs

include/lfstring.h

@@ -0,0 +1,10 @@
+#ifndef LIBFLINT_H_STRING
+#define LIBFLINT_H_STRING
+
+#include <stddef.h>
+
+size_t *find_substrings(const char* haystack, const char* needle, size_t *num_substrings);
+
+const char* substr(const char* str, size_t idx, size_t len);
+
+#endif // LIBFLINT_H_STRING