[cs] fixes for Std.is check for Float, Int and UInt (closes #2114)

- fixed broken pre-3.0 Float and Int cases for Std.is
- added optimization for Int case (check with "v is int" before calling boxing Runtime.isInt function)
- added support for UInt (it should unify with Float, right?)

my first ocaml experience, @waneck please review carefully :)