Safe stack reorganisation in terms of function casts.

Safe stack reorganisation in terms of function casts.

After some messing around this seems to work but needs
a few more tests. Working out the syntax for sk_set_cmp_func()
(cast it to a function that itself returns a function pointer)
was painful :-(

Needs some testing to see what other compilers think of this
syntax.

Also needs similar stuff for ASN1_SET_OF etc etc.