&convert on bitfield seems to be broken

[bbannier]

The following code does not compile; I would have expected the &convert to have the effect of extracting the element value.

module foo;

public type X = unit {
    x: bitfield(8) {
        a: 0..4;
    } &convert=$$.a;
};

$ printf '\x00' | spicy-dump -d foo.spicy
[error] foo.spicy:3:17-7:2: cannot coerce expression '(__bits)' of type 'tuple<uint<8>>' to type 'bitfield
(8) {
    a: 0..4; }'
[error] foo.spicy:3:17-7:2: cannot coerce expression '__parsed_x' of type 'bitfield(8) {
    a: 0..4; }' to type 'tuple<a: uint<8>>'
[error] spicy-dump: aborting after errors

[rsmmr]

I believe this will come with #1032.