Try to express the stretchy nodes (`StretchableOp`, `Fenced`, `SizedParen`) with other existing nodes

[tmke8]

Too much rendering work is happening in the emitter there. The StretchableOp type (not to be confused with Node::StretchableOp) also shouldn't be required anymore now that we have the MathML categories.

• turn OpAttr into a bitset Turn OpAttr into a bitset #865
• Node::StretchableOp Remove Node::StretchableOp #866
• Node::SizedParen
  • where to store the size?
• Node::Fenced
  • this'll just be replaced by a Node::Row with certain content