Replaced hand-written dynamic type introspection with dynamic_cast.
Alternation of 'RegExp's should preserve 'ins_access' attribute.

When one builds 'AltOp' from two 'RegExp's, one sometimes has to
break these 'RegExp's in pieces in order to merge their common prefix.
In such cases, if one of the original 'RegExp's has 'ins_access'
set to 'PRIVATE', it is lost (defaults to 'SHARED') after alternation.

This commit fixes Gentoo bug https://bugs.gentoo.org/show_bug.cgi?id=518904.
Release 0.13.7.5.
Updated version to 0.13.6.dev