purity of show -- show implementation is culture-sensitive at the moment

[alaendle]

printLn $ show 4.2

might lead to 4.2 or 4,2, depending on the current culture on which the dotnet code is executed. While this is .net default behavior of .ToString(), I'm not sure if this should be fixed with CultureInfo.InvariantCulture; maybe some code would already expect the .net default here; while on the other hand from an Idris point of view show should be pure.