Fix double std::move on objects in 'lenses::getset' on set call

[dimula73]

This is the final version of the patch that fixes both, double-move problem and extra getter call on set(lens,...) operation.

The patch ensures the following requirements:

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

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

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

2. 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 #160