ochre

Copperbox revision 2774.

I've re-implemented the tagless phantom typed layer with a state monad instead. The types are much simpler, so I've included the all important Num and Fractional instances for signals.

The new monadic layer is in a style I've never used before - previously I've had code generators that could collect statements using a monoid so the user code would use the do-notation with a Writer as the underlying monad. Here the monad builds a single expression (usually nested) - the expression has no mempty or mappend so it is built with a library of combinators that model the syntax faithfully, user code does not use the do-notation.