Promote TypeData.csx in some way

There is useful information collected in the TypeData and TypeDatas classes in the CodeWriter.csx file and it would be a shame to simply delete it.

The first thought is to translate the information to extension methods, such as IsSigned() and IsFractional() on the various numeric types, but the info is about the types themselves, not specific instances of those types. For example, ((int)1).IsSigned() and ((int)-1).IsSigned() would both return true, which gives off a definite code smell. The simplest solution would be to ammend the names, such as IsSignedType(), but while better it still smells.

Instead, maybe they should be methods on a TypeHelper static class. It would be trivial to convert TypeDatas itself into that class. This is the path I'm going to investigate.