Unit switch with look-ahead cannot be nested

[bbannier]

When branching on a value unit switch can be nested, e.g., the following works:

module foo;

public type X = unit(typ: uint8) {
    switch (typ) {
        1 -> switch (typ) {
            1 -> a: b"a";
            2 -> b: b"b";
        };
        3 -> c: b"c";
    };
};

While this would seem contrived it allows collecting related information together which reduces the need to store values if other parsing depends on this.

The same pattern does not seem to work with unit switch with look-ahead, e.g.,

module foo;

public type X = unit(typ: uint8) {
    switch {
        -> switch {
            -> a: b"a";
            -> b: b"b";
        };
        -> c: b"c";
    };

};

$ spicyc -dj foo.spicy
[error] foo.spicy:5:12-5:17: syntax error, unexpected SWITCH
[error] spicyc: parse error

[bbannier]

Also backported to releases/1.11.