This switches the detection of a templated child() method to a
concept. It's a lot nicer than my abomination...

This is a partial fix for #2 (only works for master because it requires
the concept support in dune-common).

References !5.

* carsten.graeser/dune-typetree-feature/drop-lazy-decltype:
  [cleanup] Drop _lazy_static_decltype in favour of return type deduction
  Add dummy overload to generate nice compiler errors
  Use concepts to check for the child<0>() method