Allow &synchronized with static length fields and not just literals

[sethhall]

Some protocols have static length fields before the literal where we might use the &synchronize attribute but currently spicy only allows literals before the literal where &synchronize is used.

Here is a toy parser that should illustrate what I would like to be able to do...

module test;

public type PDUs = unit {
    : (PDU &synchronize)[];
};

type PDU = unit {
    len: uint8;
    sub: subPDU &size=self.len;
};

type subPDU = unit {
    typ: /this|that/ &synchronize;
    switch (self.typ) {
        /this/ -> eight: uint8;
        /that/ -> thirty_two: uint32;
        *      -> : bytes &eod;
    };
};

This currently give this error....

[error] ../test13.spicy:9:10-9:14: &synchronize cannot be used on field, look-ahead contains non-literals
[error] spicyc: aborting after errors