Add fuzzer for mb_convert_encoding

This uses the php-fuzz-mbstring name, moving the existing fuzzer
to php-fuzz-mbregex.

Generate tracing jit corpus in generate_all.php

Using same corpus as function jit. To allow oss-fuzz integration.

Generate function-jit corpus in generate_all.php

For use by oss-fuzz.

Generate execute corpus in generate_all.php

And add crypt() to the function blacklist, it can be very slow.

Extract some common fuzzer code