Refactor rendering approach

[colinodell]

There are several rendering-related ideas I'd like to explore for v3:

• [ ] Avoiding recursion (parsing and AST iteration already avoid recursion)
• [ ] Using an OutputWriter object
• [ ] Investigate performance of writing to a php://memory stream instead of concatenating strings
• [ ] Allowing renderers to determine what to render (à la Symfony's NormalizerInterface) so that renderers could potentially handle multiple node types without needing to tell the Environment in advance
• [ ] Better support for multiple render formats (html, md, xml)
• [ ] Simplifying the creation of custom renderers (see #967)

Whether these actually get implemented will depend on how the testing goes.