Add full_type_id() to yield fully-qualified type name

[rhaschke]

I found it handy for debugging to print the fully qualified type name, including reference specifiers.
This code originates from https://stackoverflow.com/questions/81870/is-it-possible-to-print-a-variables-type-in-standard-c

[YannickJadoul]

I'm not sure adding a dead code, for debugging purposes only, is a great idea from a maintenance point of view.

(Also, pybind11 currently seems to be using west-side const. Let's not get into that discussion, but this would be inconsistent.)

[rhaschke]

Understandable. Nevertheless, using full_type_id() instead of type_id() might become handy in some existing unit tests or debug outputs of pybind11 as well, particularly if the reference specifiers are relevant.
But don't mind to reject this PR. I just wanted to propose it, because I have it lying around.

[YannickJadoul]

Just my two cents, though. Others can chime in.

If this is would actually be used somewhere, that's a different story.