Tutorial 4: Types, containers, and errors - init (#7)

* Init from MarekSuchanek/FPCourse (improved)

* Basic improvements of old texts

* Boxed and unboxed types

* Strictness with types

* Basic strings and overload intro

* String types done

* Containers briefly explained

* Improved undefined and try-catch info

* Grammar fix with Grammarly

* revisions

* Maybe and Either examples

* Simplified types performance theory

* Fixed Either example

Author: MarekSuchanek