Improve string parsing code

Improve string parsing code

Currently next_string and next_hexstring functions use a static
buffer in the iterator to store the value.  This value is clobbered
as soon as next_string or next_hexstring is called.  Instead,
we copy the entire line in iter_next and use it as a scratch buffer.
The only limitation is that lines of max 2048 are possible, however
these are limited to around this size by parts of the standard.