[formatters] Add a deque formatter for libstdcpp and fix the libcxx one

This adds the formatters for libstdcpp's deque as a python
implementation. It adds comprehensive tests for the two different
storage strategies deque uses. Besides that, this fixes a couple of bugs
in the libcxx implementation. Finally, both implementation run against
the same tests.

This is a minor improvement on top of Danil Stefaniuc's formatter.