Quadratic parsing of nested image wikilinks with `ENABLE_WIKILINKS`

With Options::ENABLEWIKILINKS, handlewikilink builds a wikilink's target from the raw text of its body: scanwikilinkpipe walks it looking for |, then contains'\0' and fromreplacenuls walk it again. Once a wikilink is created, disablealllinks only disables Link entries, so every enclosing ! stays...