Sketches in code.world/blocks

I have now implemented sketches in https://code.world and https://code.world/haskell. It isn't yet integrated into https://code.world/blocks. This requires adding the blocks themselves, which should be pretty simple, and also modifying the code generator to add the right import when they are used.

An outstanding implementation might also include the sketch itself (which is an SVG file) on the block, so it's easier for kids using blocks to browse through the set of pictures available.