`autoconf && autoheader' is sped up. Now, speed up `autoheader &&

`autoconf && autoheader' is sped up.  Now, speed up `autoheader &&
autoconf', i.e., in addition to caching traces, cache the output.
* bin/autom4te.in (Request::cache): Rename as...
(Request::id): this.
($cache, $icache, $tcache, $ocache): New.
(&handle_m4): Save M4 output in the cache instead of $tmp.
(&handle_output): Adjust.
(&up_to_date_p): Check that the output cache is up to date too.
(top level): Run `&handle_m4' iff force or the cache is invalid.
Run `&handle_output' if the output cache is more recent.