Add support for higher-rank polymorphism

This merge request adds support for higher-rank polymorphism to the parser. Furthermore a new language extension RankNTypes is introduced. The representation of AbstractCurry is also adapted to support higher-rank polymorphism.

The recognized syntax of Curry will change as follows (only the relevant parts are listed):

\begin{aligned} \cdots \end{aligned}

\begin{aligned} \textit{ConstrDecl} &::=\dots\\ &\phantom{::}\mid\phantom{!)}\textit{AppTypeExpr}~\textit{ConOp}~\textit{AppTypeExpr}\\ &\phantom{::}\mid\phantom{!)}\dots\\ \textit{ForallVarID} &::=\textit{Ident}\\ \textit{ForallVars} &::=\texttt{forall}~\textit{ForallVarID}_{1}\ldots\textit{ForallVarID}_{n}~\texttt{.} &&(n\geq 1)\\ \textit{TypeExpr} &::=\{\textit{ForallVars}\}~[\textit{Context}~\texttt{=>}]~\textit{FuncTypeExpr}\\ \textit{FuncTypeExpr} &::=\textit{AppTypeExpr}~[\texttt{->}~\textit{TypeExpr}]\\ \textit{AppTypeExpr} &::=[\textit{AppTypeExpr}]~\textit{SimpleTypeExpr}\\ \textit{Signature} &::=\textit{Functions}~\texttt{::}~\textit{TypeExpr}\\ \textit{Expr} &::=\textit{InfixExpr}~\texttt{::}~\textit{TypeExpr}\\ &\phantom{::}\mid\phantom{=}\dots \end{aligned}

\begin{aligned} \cdots \end{aligned}

\begin{aligned} \textit{QualTypeExpr} &::=\dots &&(\text{removed})\\ \textit{TypeAppExpr} &::=\dots &&(\text{removed}) \end{aligned}

\begin{aligned} \cdots \end{aligned}