core: replace `MultiMap` with standard library container

[mkviatkovskii]

Motivation Currently Indigo uses own-written implementation of MultiMap instead of standard one. It does not contain any unique API that could not be replaced with standard container. Also it uses red-black trees even if we don't need the order of the keys, so we could also achieve performance improvement here. The implementation could be found at core/indigo-core/common/base_cpp/multimap.h.

ToDo

1. Find all places in code that use MultiMap<K, V>, check if K is trivially hashable and order of elements is required, and replace it with std::multimap<K, V> or std::unordered_multimap<K, V> depending on the situation.