DSLX: alias required for `::MAX` attribute on array-style bits types

With the soon-to-land attribute grammar you cannot write: `uN[128]::MAX`, you must make a type alias and take `::MAX` on that, e.g. `type MyU128 = uN[128]; MyU128::MAX`.

(Of course for all the types have a direct builtin name we _can_ take `::MAX` on those, e.g. `u32::MAX`)

This limitation could be lifted by expanding the grammar.