`uri_parser_process()` takes a `const char *` as input parameter but
the result is a struct of `char *` to the original string.
This may lead a user to modifying the strings in `uri_parser_result_t`
which will cause a crash if the original string resides in read-only
memory (and violates the no-modifications promise of the const parameter
in `uri_parser_process()`).
To fix this, make the resulting strings `const` as well, so nobody dares
to touch them in a writing way.
