Make BijectiveMap compile on more machines by avoiding const

[haved]

When I originally wrote BijectiveMap, I wrote the iterator interface as a thin wrapper around the forward mapping's cbegin() and cend().

The iterator you get from cbegin() has the reference type const std::pair<const Key, Value> &.
To achieve some symmetry, I added an additional const to get const std::pair<const K, const V> &.

Later it turned out that a std::pair or std::unordered_map with added const behaves very unergonomically. When the STL gives a std::pair<const Key, Value> it almost feels like an ugly hack to make only the first immutable. Trying to add additional const for symmetry only makes the assignment operator break in weird ways, and only on some systems.

In practice all I want is for the iterator to return const std::pair<Key, Value> &, but there is no subtyping of these ugly types, so the best I can do without extremely cursed and maybe illegal casting is const std::pair<const Key, Value> &.