Context API: support multiple contexts

[nikochiko]

Our global context being assumed as the only context in a program puts several limitations on the project:

• work with parallel contexts: e.g. render both an SVG and a PNG
• switch between contexts: this could be very helpful when working in a REPL - when you want to work on a clean slate, but also don't want to lose the context that you have so far worked on

More than this being an actual use case, I feel that the current API unnecessarily makes these things impossible, given that other than show, all other methods that use the global context are only used internally.