Cannot use unit parameters to specify a literal to parse

[bbannier]

When parsing a field one can use e.g., const values to specify literals to expect, but using unit parameters in the same place does not resolve, e.g.,

module foo;

type X = unit {
    : Y(0);
};

type Y = unit(expected: uint8) {
    : expected;
};

$ spicyc -dj foo.spicy
[error] foo.spicy:8:5-8:15: unknown ID 'expected'
[error] spicyc: aborting after errors

[rsmmr]

The error message isn't great but it's correct to reject this: these constants/literals must be know at compile time, so parameters can't be supported.