Allow use of `&synchronize` on unit switch fields

[bbannier]

We currently do not allow the use of &synchronize on unit switch fields, e.g., the following is not possible:

public type X = unit {
    switch {
        -> a: b"A" &synchronize;
        -> b: b"B" &synchronize;
        -> c: b"C" &synchronize;
    };
};

We should consider allowing this if

1. all cases &synchronize

2. or maybe the switch itself is &synchronize, e.g.,

   public type X = unit {
       switch {
           -> a: b"A";
           -> b: b"B";
           -> c: b"C";
       } &synchronize;
   };