helper/function

+ Add dake/helper/function.hpp for small helper functions.
+ Add sum(), difference(), product(), quotient(), conjunction(),
  disjunction() and exclusive_disjunction() as wrappers for the
  respective operators to dake/helper/function.hpp.
Make header functions static inline

! Functions defined in header files should be static (and probably also
  inline).
Shorten header function aliases

+ Shorten the header functions which only provide easy access to the
  real (more complex) ones.
container/algorithm: map

+ Add map() to container/algorithm.