Add support for creating JavaScript functions/callbacks/code in Python

This is really naive, preliminary approach, but allowed to port `server/animated` and run it without server (see `glyphs/roll`). This will be used later by widgets, but has value on its own. Later it should be possible to use this with hover, click, etc. events, e.g. `glyph.on_enter = Function(self, Set(self.color, self.color.lighten(0.1))) ` (that's very rough pseudo-code).