[dune-functions] Type erasure

[Carsten Gräser]

Hi,
I just had a look at Adobe::Poly, a library that helps to build
interfaces based on type-erasure. Interestingly it seems to be
very close to our current approach where one can compose
such an interface from three ingredients:

* the internal, polymorphic interface,
* the internal parametrized implementation of this interface,
* the public non-polymorphic interface forwarding to the internal one.

They also provide small buffer optimization but it seems
that their default buffer size is 16.

Another approach I found is boost::TypeErasure also targeting
the same purpose. However this seems to much more heavy weighted
with a lot of magic happening.

Best,
Carsten