Core changes for NavigableTOC extension:

Core changes for NavigableTOC extension:
* Always generate the section tree, even when we're not generating a TOC
* Add Parser::mergeSectionTrees() to merge two section trees into one
* Add Linker::generateTOC() to generate the HTML for a TOC from a section tree, and add the section anchor to the section tree to facilitate this. This adds the ability to generate TOCs in extensions; haven't converted Parser.php to use it (yet?). As a side effect, this fixes API bug 18720