Remove public remove public modifier for public functions in the standard library

[jurgenvinju]

Users take the code of the standard library as exemplary rascal code.

Long time ago we switched the default for functions from private to public, for the sake of usability and brevity. We have never removed the modifier in existing library code.

With more and more new functions being added this starts to become inconsistent and therefore a bit confusing.

Let's add private where it was forgotten and remove public where it's unnecessary; everywhere in the stdlib.

Typically something to do after merging rascal-core back into rascal.