Consider "Read" and "Show" constraints when defaulting
Currently, only Eq and Ord constraints are considered when a numeric constraint is defaulted to Int or Float.
This way, expressions like read "1.0" :: (Read a, Fractional a) => a cannot be entered in the REPL without getting an error.
To fix this problem, Read and Show constraints should be handled the same way as Eq and Ord constraints are handled.