Add `%Ash.Expr{}`, which will be returned from `expr/1`

Its not possible to easily tell “did they pass an expression”. You can use `Ash.Expr.expr?` but that literally traverses the entire value.

I *think* we can actually change this transparently to end users, since they shouldn’t be manipulating expressions, but we’ll need to adjust a bunch of code to just pass the contained expression by. This will also be useful in disambiguating interfaces.