Add a representation for function arguments

[pepeiborra]

The DebugTrace datatype does not provide an encoding for functions, so the Hoed backend has been representing them as strings, for instance:

$arg1 = { \ 'l'  -> True, \ 'l'  -> True}

Having a representation would allow the frontend to render these more effectively (and choosing when to display them), and would increase sharing, making the trace more compact.

[pepeiborra]

We might want a representation for lists as well, for the same reasons.