First `Patch` version.

First, I must say that I've used `Patch` instead of `Let`, since the
latter is a Haskell reserved word.

A `Patch` action has been added. It receives the entity, the lens that
points to the part and finally the new value to set in the part.

The interpreter is a mess. It has been almost a lottery to find a
valid version, since the compiler was experiencing troubles to infer
the valid types for v and v2. That led me to return the whole value
instead of void.

Finally, I've added a new application, a dummy formulary to test that
the changes are truly achieved.