Basically, we shouldn't calculate the getter when we know that the
value will be dropped.

The patch partially fixes arximboldi#160, because now the getter is not called
at all during the set operation. But the double-move can still
happen when both, getter and setter calls are performed.

error: 'auto' not allowed in function prototype

The patch ensures the following requirements:

1) When `view(lens, whole)` is called:

   * the setter lambda is neither called nor constructed.
     It avoid unnecessary copies of the 'whole' object.

   * the `whole` object is directly forwarded into the getter lambda,
     without any copies.

3) When `set(lens, whole, part)` is called:

   * no getter is called at the toplevel lens (which
     value was previously discarded).

   * on the lower lenses, the getter receives an 'lvalue'
     reference to the `whole` object, and setter receives an
     'rvalue' reference to the `whole` object. It ensures
     that there is no double-move problem.

Fixes arximboldi#160

When the moved-from object is assigned, it becomes valid again. None
of our code seems to use it though.

Such lens causes:

1) Compilaion failure due to the missing const functor in getset_t
   (also fixed in the patch)

2) A crash caused by double-move in the attr implementation
   (cannot be fixed without rewriting attr into getset)

The previous implementation cased severe issues:

1) Double-move of `p` argument when passing a prvalue into a
   nested chain of two attr lenses, which usually causes a
   crash.

2) `f = LAGER_FWD(f)` creates a `const decltype(f) f` object
   inside the lambda, which prevents the value to be perfect-
   forwarded with `identity_functor` and `const_functor` when
   setting/fetching it.

The functor should be forwarded into the lens.