Kill the reuse-by-shared-name pattern

There are several places where JavaScript code relies on the pattern of calling a function, and assuming that some other referenced JavaScript will define it. For example, `clearCode` and `updateUI` and `compile` are defined separately in codeworld.js and funblocks.js, and which one is called depends on which JavaScript code is loaded into the containing page.

That's an awful pattern. Code should be reorganized so that dependencies are passed in to the relevant functions, not picked up from the global namespace.

See #66