Aka: how did this ever work...

[string] rename internal::{from,to}_string

Using `from_string` and `to_string` in user code inside a
`namespace internal` failed, since our variants in
 `Dune::XT::Common::internal` had the same names as the one in
`Dune::XT::Common`, but different signatures. This should fix that issue.

-this is due to GCC bug 53431

fix warnings about double ;; in gdt

remove redundant line

- don't know exactly why it works like this, I suspect it is
due to the DEPENDENCYCHECK dune/xt/common/test/CMakeLists.txt
that depends on the xtcommon variable defined in the header
listing

- the DUNE_UNUSED macro is not a function, this lead to
some remaining warnings about unused variables

this closes #17

to avoid them gettin gheader-checked on their own