Update dependencies for containers-0.5 and mtl-2.1
Currently, arithmoi has upper bounds that drag down the versions of mtl and containers. New versions of these packages were released recently, and arithmoi builds fine with them (no errors or warnings).
Two upstream changes I'm aware of:
In mtl, the MonadReader, MonadWriter, and MonadState classes each have an optional method for lifting a pure reader, writer, or state function to the monad. This is mainly for performance.
In containers, Data.IntMap and Data.Map now have strict and lazy variants of their methods.