Operations on context returning unit

[jacereda]

Should there be operations on context returning unit?
Starting with 0.11, operating on the canvas becomes a bit uglier due to the explicit discard:

_ <- Canvas.scale s ctx

Perhaps we could have things like:

Canvas.scale_ s ctx

and keep the current operations as they are to allow chaining.

[gabejohnson]

Would a better option be to add a Discard instance for Context2D?

[gabejohnson]

My reasoning is that ctx is mutable so we don't really care about the return value anyway.

[hdgarrood]

Resolved by #53.