GC: Bugfix in visit_short_svalue to avoid fatal.

Fixes a fatal that was triggered if Pike.count_memory or Pike.identify_cycle
was performed on a backend object having either pending call_outs or any
active or inactive fd:s (it seems that's the only call site of
gc_recurse_short_svalue, which in turn is the only call site of
visit_short_svalue).
Documentation update.
ASN1 compatibility
No need for COMPATIBILITY now that we have __deprecated__. Removed the old class aliases and made more internal things protected.
No need for COMPATIBILITY now that we have __deprecated__.
Build: Support compilers with union aliasing problems.

There are apparently compilers that don't understand that the
fields in unions overlap, and overcache overwritten fieldvalues.

Also restores support for compilers without support for static union init.