Setup clang-format code beautification

[stevengj]

The best tool I can find for automatic beautification (indenting, brace placement, etcetera) of C++ code is clang-format, which seems to be the most modern and well-maintained open-source tool for this right now

This PR doesn't actually run clang-format yet (which will cause conflicts on every other PR), but it prepares the way, via:

• A .clang-format file in the top-level directory sets the default format when you run clang-format.
  The clang-format defaults (LLVM style, I think) seem pretty reasonable, with a couple of tweaks.

• For it to format our LOOP_IN_FOO and FOR_FOO looping macros correctly, the loop bodies must be enclosed in braces. That is the cause of most of the changes in this PR.

• We don't want to run clang-format on the SWIG-generated files (mainly because it is very slow), so I renamed them to *.cxx to easily distinguish them from hand-written code

You can just run find . -name '*.*pp' | xargs clang-format -i in the source tree to automatically fix all of the code spacing.

Once we actually run clang-format and check into master, any existing PR will get lots of conflicts. You might be able to do git rebase -Xignore-all-space master to mostly fix this. Alternatively, you could run clang-format on your files and rebase with the reformatted files. Could be a little bit of a pain for large PRs.

cc @ChristopherHogan

[stevengj]

See also this article on rebasing branches after running clang-format: https://engineering.mongodb.com/post/succeeding-with-clangformat-part-3-persisting-the-change … they have a suggested procedure and even a Python script, so hopefully this won't be too difficult.

[stevengj]

Looks like I misplaced a brace in boundaries.cpp.