[PATCH 00/13] Convert notmuch show to use structure printers

[Austin Clements]

This patch series converts notmuch show to use the new structure
printer infrastructure, cleaning up the code and paving the way to
easily support other structure types.  There are a lot of patches, but
most of them are small and the conversion is very mechanical.

The first three patches set up prerequisites by making the test
infrastructure more resilient to irrelevant JSON changes and by
introducing a new method to the sprinter.  The next three patches
introduce sprinters to notmuch show and feed them through all of its
layers, though they remain unused.  The remaining patches work their
way back up from the leaves of the JSON printer all the way to
do_show, converting each layer to use the sprinter.  By doing this
conversion from the leaves of the structure up, the output remains
valid and the tests continue to pass at every step.

The full series has been tested with both GMime 2.4 and GMime 2.6.

The first two patches together and the third patch alone stand on
their own and could be pushed before the rest of the series if
desired.