vector: Additional string vector definitions.

ast_vector_string_split:
This function will add items to an ast_vector_string by splitting values
of a string buffer.  Items are appended to the vector in the order they
are found.

ast_vector_const_string:
A vector of 'const char *'.

Change-Id: I1bf02a1efeb2baeea11c59c557d39dd1197494d7