Reduce Callback.h using C++11

The bulk of Callback.h was auto-generated as 6 specialisations, handling
zero to five arguments.

This can now be handled without specialisation using C++11 variadic
templates, reducing the file from 4,900 lines to 900 lines.

This should reduce compilation time, and offset potential increases from
use of `<type_traits>` or a local `mbed_cxxsupport.h` equivalent.

Several other improvents to `Callback` are possible and/or desirable
with C++11, such as the ability to store lambdas, but this commit is
purely the variadic simplification.

Author: kjbracey