Moves main file to bin folder and rename it

* refactors lib setup, smallstep is part of a crate that is named computation
Adds do-nothing and assign nodes

* prepares for Assign node to work correctly
* adds tests for both nodes
Refactors string argument for some Nodes

* instead of `str` type the String class is used
Adds an insert function to Environment
Removes unnecessary mod reference
Implements reduce function for Assign node

* adds test to reduce an Assign node to a `do-nothing` Node
Adds a sample to binary smallstep
Adds tests to check environment after assignment
Environment implements Show trait, Machine does output environment as well
Merge pull request #3 from justahero/extend-with-assign-statement

Extend with assign statement