lib: add file universe, describing universe and how it may be used

[michaellilltokiwa]

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[maxteufel]

The documentation generator should probably consider a special case for this and include this file as documentation for the universe feature, otherwise this is a bit useless.

[michaellilltokiwa]

@maxteufel Yes, good idea.

[fridis]

Since we define a number of features in the universe, e..g., type_of or infix =, I think it is good to have this file, such that we have a default place to put such features if there is no other good place. Not suggesting to move any of these, though.