Add "let"-expressions to language

[AraneusRota]

• A "let"-ADT (case class)
• Use macros to detect usage of standard Scala variables
• Store all "state" implicitly in every result so that no self implemented let or control flow expressions are needed. All of Scala's functionality can be used (let, if, loops, ...) (done in FunctionalTape)

To differentiate programs we need a language that not only consists of mathematical terms. Firstly it needs variables implemented with "let"-expressions. Two different approaches are considered: A "let"-ADT or standard Scala variables with macros

Consider: Shadowing of variables when implementing "let" as ADT