Update all cc code using .clang-format; please do so as well for future PRs
etc.; emacs has a handy 'clang-format' mode to make this automatic.
For comparing old changes with git blame, we can disregard this one using
--ignore-rev
(see https://www.moxio.com/blog/43/ignoring-bulk-change-commits-with-git-blame )
Using deque gives compilation errors when compiling on
MacOS/clang (where it defaults to libc++ rather than gcc's libstdc++)
```
#include <deque>
struct Foo { std::deque<Foo> foos; };
int main() { Foo foo; }
```
So, let's use a vector instead; this is a drop-in replacement here, but
unfortunately in some future code...
Seems there are problems compiling mu with XCode 11.6 (see build tests);
apparently because of libc++ being different from libstdc++.
clang++ builds works fine as long as we're using libstdc++.