Add std::initializer_list to the following containers:

* deque
* map
* multimap
* set
* multiset
* list
* slist

Tested on clant and g++
add std::initializer_list support for flat_map and flat_multimap
add std::initializer_list support to flat_set and flat_multiset
Fixed compilation error in C++03 compilers and updated changelist with initializer_lists.
Merge branch 'matekm-initializer_list_support' into develop