Preserve insertion order in objects. Closes #169.
Support more arguments for defs
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
Use uint32_t for codepoint in  jv_string_append_codepoint()
Add explode/implode jq functions to match jv API
Add startswith/endswith
Add string multiplication by number
Test starts/endswith and string multiplication
Add string division by string (split on separator)
Add tests for string division/splitting
Add string slicing
Test string slicing
Document string functions and slicing
Document string multiplication and division
Document split function
Add tojson and fromjson builtins
Test tojson and fromjson
Document tojson and fromjson builtins
Add -e | --exit-status CLI option
Document -e / --exit-status argument