remove constructor type reference from the language

[jurgenvinju]

Describe the bug

Statemenf.if x = [Statement] "if x then y else z fi";

and

if (Statement.if _ := [Statement] "if x then y else z fi") {
   ...
}

1. I bet this is never used
2. documentation contains obscure error message: https://new.rascal-mpl.org/docs/compiletimeerrors/ambiguousfunctionreference/ which is wrongly documented

Constructors are types in vallang, but for no good use. Perhaps this was a unification step too far in 2007/2008. In Rascal constructors are not used as types. Constructor functions are used as functions, and they have a function type, and constructors are used to create constructor trees of the algebraic datatype they belong to.

So the proposal is to remove this feature entirely:

1. From the interpreter, throwing errors when it is used (not implemented)
2. From the syntax definition of Rascal itself
3. From vallang by changing class ConstructorType extends Type to class Constructor.
4. The final step would also align vallang semantics with the semantics of the Type and ParseTree standard library modules.

[jurgenvinju]

• We can start by removing this document: https://new.rascal-mpl.org/docs/compiletimeerrors/ambiguousfunctionreference/
• We can delay the changes in vallang as long as we want because they would break the interpreter and the compiled run-time.