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 Simplify custom renderers #967)

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