improve const correctness for patricia tree maps
improve const correctness for patricia tree maps

Summary: This patch marks the dereference operator on patricia tree map iterators as `const`.

Reviewed By: minjang

Differential Revision: D22490729

fbshipit-source-id: a4afacb7396c0ceb267a4c30520b48835ae275dc

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex