[pattern-switch] Opting into totality

[Brian Goetz]

> Given that if there is a default it's already a sealed switch and that 
> i can add a default to make it a sealed switch,
> i struggle to see where to use a classical statement switch and where 
> to use a sealed switch ?

It feels like we're going in circles :)

One point here is that total switches are generally better _without_ 
default clauses, if that is semantically practical (e.g., enums, sealed 
types, total type patterns) because then you don't have a catch-all that 
sweeps mistakes under the rug.  But the there needs to be a way to 
engage totality checking / handling for statements.

Totalizing with default should not be your first move.