The ::value_type of the std::map/std::multimap/std::unordered_map
containers is std::pair<const Key, T>. Dropping the const results in an
unnecessary copy, for example in C++11 range-based loops.
For this I started with a more general scripted diff, then narrowed it
down based on the inspection showing that all actual
map/multimap/unordered_map variables used in loops start with m or have
map in the name.

-BEGIN VERIFY SCRIPT-
sed -i -E 's/for \(([^<]*)std::pair<([^c])(.+) : m/for
(\1std::pair<const \2\3 : m/' src/*.cpp src/**/*.cpp
sed -i -E 's/for \(([^<]*)std::pair<([^c])(.+) : (.*)map/for
(\1std::pair<const \2\3 : \4map/' src/*.cpp src/**/*.cpp
-END VERIFY SCRIPT-

make check