More conversions and lots of tests

Not everything works yet. I mainly pursued low-hanging fruit here. At least most of the tests should be somewhat sound and provide a starting point for what's wrong.

PowerShell (again) doesn't adhere to the spec everywhere. And the spec is somewhat contradictory in places, too.
Maybe decimals work in PS? (Haven't tried this). Implementation is pretty easy, because a conversion to int/float by `LanguagePrimitives` should do the proper rounding.
I think you could easily check above if `rawValue().GetType().IsNumeric()`, convert to Int64 (but with `LanguagePrimitives`, not with `Convert`) and use the result. Then all numeric values should be convertible to char.