Simplify Node::eachChild #simplify
Add Fragment::postOrder

By moving it to Fragment, and then having Node::postOrder call it just
like Node::{each,fold}Child call Fragment::{each,fold}.
MathElement::finalizeInsert => finalizeInsertingMath()

...which takes in a Fragment instead of a Node, simplifying
LatexFragment::createLeftOf() and Controller::writeLatex().
Get rid of MathElement #cleanup