Implemented the cpr_string_push_back() function.
Implemented the cpr_string_pop_back() function.
Implemented the cpr_string_clear() function.
Implemented the cpr_string_data() function.
Implemented the cpr_string_resize() function.
Implemented the cpr_string_front() and cpr_string_back() functions.
Implemented the cpr_string_free() function.
Consolidated structure typedefs into the <cpr/types.h> header file.

This avoids having to duplicate the typedefs in the header files for
every container type that might include elements of another type.
Implemented the cpr_vector_of_strings() function.
Implemented the cpr_vector_of_vectors() function.
Implemented an initial trivial dynamic string example program.
Implemented the cpr_string_reserve() function.
Reordered header file function declarations in alphabetical order.
Implemented the cpr_string_at() function.
Implemented the cpr_string_append_char() function.
Implemented the cpr_string_compare() function.