Replace `[all]` with `_`

[mlhaufe]

Given that a Data declaration requires variants to be ProperCase, _ can be used in Trait declarations without ambiguity:

const fib = Trait(undefined, {
  [all]: (n) => n == 0 ? 0 :
                n == 1 ? 1 :
                fib(n-1) + fib(n-2)
})

becomes:

const fib = Trait(undefined, {
  _: (n) => n == 0 ? 0 :
                n == 1 ? 1 :
                fib(n-1) + fib(n-2)
})