Blocks should allow partial application

[mjstahl]

Currently if a lookup fails it will return Nil as the result. While in a block this should not be the case. This is how a failed lookup currently evaluates:

>>> {| x. y | x + y } value: 1
=== Nil

It should evaluate as follows:

>>> {| x. y | x + y } value: 1
=== Block

Where Block contains the expression.

{| y | 1 + y }

[mjstahl]

This was changed from bug to feature because this is an implementation of partial application for blocks.