Fix printing of variant types #56
Currently we use the code
and print_datatype (Datatype.Type tyconstr, tyargs) = string tyconstr ^//^ separate_map space print_type_atom tyargs
but when the variant doesn't expect argument we print a useless whitespace after the name of the constructor.
Suppose for instance that we define the usual bool variant then we'll have this printing
True<bool > instead of
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