Concrete syntax patterns

[rodinaarssen]

I've implemented concrete syntax patterns in the context of Concretely, which means "it's on a branch" and "works on my machine" as this is not currently integrated into the interpreter (or compiler). After @jurgenvinju finishes his work on concrete syntax in the compiler, this can be explored further.

[jurgenvinju]

shall we put the code that revolved anbout concretely on a branch and remove it from main until it works? Because now it is confusing clair users with lots of code that does not work yet.

[rodinaarssen]

Sounds like a good plan