Add jv string utility functions

Add jv string utility functions

    jv_string_empty()
        -> return an empty string with given allocated length (for fast
           appends)
    jv_string_append_codepoint
        -> append a single codepoint (int) to the given string
    jv_string_explode
        -> return an array of codepoints making up a string
    jv_string_implode
        -> return the UTF-8 encoding of an array of codepoint numbers