Currently there is no easy way to add support for different structured formatters (like JSON). For example, adding support for S-Expressions would result in code duplication. This patch series amends the situation by introducing structured formatters, which allow different implementations of structures like lists, maps, strings and numbers. The new code in sprinter.h and sprinter-json.c can be used instead of the current ad-hoc output in all parts of notmuch, a patch for notmuch-search.c is included. In a later patch, all other parts of notmuch should be adapted to the structured formatters, and the creation of formatters should be centralised (to make adding new formatters easier). A "structured" formatter is provided for notmuch-search that prints the current text format. This removes all special-casing from notmuch-search.c, only structured output is used (and the formatter discards unnecessary parts or introduces extra formatting).