The `gen` program was causing a lengthy single-threaded pause in
the BIND build. When generating RDATATYPE_FROMTEXT_SW(), `gen` hit
the inner loop of `find_typename()` over 1.2 billion times. This
change avoids long deeply-nested loops, so `gen` now runs in less
than 10ms, about 300x faster.
No changes to the output.
96b6d78f75