Changes for 1.56 made during Git transition:

*  Improved Doxygen generated reference and updated and fixed forward-declaration header.

*  Source breaking: Removed previously deprecated `xxx_dont_splay` functions from splay containers and removed `splay_set_base_hook` and `splay_set_member_hook`from splay containers.
Fixes #9520
Fixed documentation after removal of deprecated splay_set_xxx_hooks.
pack_options is now public, documented and tested so that it can be used by other libraries like Container, to specify new options for customizable containers.
Simplified rebalancing code extracting common operations to a local variable
Fixed copy/paste error in "cache_last" option's documentation.
Simplify splay algorithms avoiding redundant calls.
intrusive changes for WinRT support