The typenum for non-structured types is still accessible at ::value,
and the dtype object for all types is accessible at ::dtype().

The format strings that are known at compile time are now accessible
via both ::value and ::format(), and format strings for everything
else is accessible via ::format(). This makes it backwards compatible.

Add pybind11::make_key_iterator for map iteration

This allows exposing a dict-like interface to python code, allowing
iteration over keys via:

    for k in custommapping:
        ...

while still allowing iteration over pairs, so that you can also
implement 'dict.items()' functionality which returns a pair iterator,
allowing:

    for k, v in custommapping.items():
        ...

example-sequences-and-iterators is updated with a custom class providing
both types of iteration.