Bind the other predecessors when matching on more than one number

[Nicolas Abril]

Is your feature request related to a problem? Please describe.
If we do

switch x:
  case 0:
  case 1:
  case _:

In the last case the variable x-2 is bound. However, x-1 is not and users must either do x - 1 or x-2 + 1, which is counterintuitive.

I'd like to be able to write this fibonacci function

fib n = switch n { 0: 0; 1: 1; _: (+ (fib n-1) (fib n-1))

Describe the solution you'd like
We should bind the other predecessors using a use term. In the fib example above, with should convert the code to this

fib = @n switch n {
  0: 0
  _: switch n-1 {
    0: 1
    _: @n-2 use n-1 = (+ n-2 1); (+ (fib n-1) (fib n-2))
  }
}

Describe alternatives you've considered
Keeping things as they are would also work, just would be slightly more annoying for users in some cases.