Avoid repeatedly scanning the list of extensions

Because extensions were keyed by type which is sparse, we were continually
scanning the list to find the one we wanted. The way we stored them also
had the side effect that we were running initialisers/finalisers in a
different oder to the parsers. In this commit we change things so that we
instead key on an index value for each extension.

Perl changes reviewed by Richard Levitte. Non-perl changes reviewed by Rich
Salz

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>