Rewrite string_to_hex() and hex_to_string() using CBB/CBS

Rewrite string_to_hex() and hex_to_string() using CBB/CBS

These helpers used to contain messy pointer bashing some with weird logic
for NUL termination. This can be written more safely and cleanly using
CBB/CBS, so do that. The result is nearly but not entirely identical to
code used elsewhere due to some strange semantics. Apart from errors pushed
on the stack due to out-of-memory conditions, care was taken to preserve
error codes.

ok jsing