OpenSystemsDevelopment / qpdf

This reverts commit dc059560e73e0b373a6e54e71b07e3af4b692cb4.

I changed my mind. There's no harm in leaving it deprecated for a
release cycle.

See ChangeLog for rationale for not deprecating it as originally
planned.

I had some ideas about some more convenience methods from discussions
with some developers, but I decided that the newly added ones cover
most of the use cases. The other ideas were too hard to explain
clearly and therefore too specialized to put into the public API,
where I would have to support them for a long time.

Where not possible, use "auto" to get the iterator type.

Editorial note: I have avoid this change for a long time because of
not wanting to make gratuitous changes to version history, which can
obscure when certain changes were made, but with having recently
touched every single file to apply automatic code formatting and with
making several broad changes to the API, I decided it was time to take
the plunge and get rid of the older (pre-C++11) verbose iterator
syntax. The new code is just easier to read and understand, and in
many cases, it will be more effecient as fewer temporary copies are
being made.

m-holger, if you're reading, you can see that I've finally come
around. :-)

Change .clang-format and commit automated changes from a fresh run of
format-code

It was being run at configuration time, not build time.

Make it so that a key with a null value is always treated as not being
present. This was inconsistent before.

Character transcoding from Unicode to single-byte characters used
hard-coded switch statements because the code predated our adoption of
C++11. Now we have thread-safe, static initialization of map literals,
so use that instead.

I was using AttributeMacros incorrectly, and a recent update to
clang-format broke the formatting.

Remove the name/number tree object helper constructors that don't take
a QPDF&.