Add support for fixed values

[SebastienGllmt]

Supposed the following CDDL

bootstrapEraDistr = 1

Expected code

#[wasm_bindgen]
#[derive(Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
pub struct BootstrapEraDistr {
}

#[wasm_bindgen]
impl BootstrapEraDistr {
    pub fn new() -> Self {
        Self {
        }
    }
}

impl Deserialize for BootstrapEraDistr {
    fn deserialize<R: BufRead + Seek>(raw: &mut Deserializer<R>) -> Result<Self, DeserializeError> {
        (|| -> Result<_, DeserializeError> {
            let len = raw.array()?;
            let mut read_len = CBORReadLen::new(len);
            read_len.read_elems(1)?;
            let ret = Self::deserialize_as_embedded_group(raw, len);
            match len {
                cbor_event::Len::Len(_) => (),
                cbor_event::Len::Indefinite => match raw.special()? {
                    CBORSpecial::Break => (),
                    _ => return Err(DeserializeFailure::EndingBreakMissing.into()),
                },
            }
            ret
        })().map_err(|e| e.annotate("BootstrapEraDistr"))
    }
}

impl DeserializeEmbeddedGroup for BootstrapEraDistr {
    fn deserialize_as_embedded_group<R: BufRead + Seek>(raw: &mut Deserializer<R>, len: cbor_event::Len) -> Result<Self, DeserializeError> {
        (|| -> Result<_, DeserializeError> {
            let index_0_value = raw.unsigned_integer()?;
            if index_0_value != 1 {
                return Err(DeserializeFailure::FixedValueMismatch{ found: Key::Uint(index_0_value), expected: Key::Uint(1) }.into());
            }
            Ok(())
        })().map_err(|e| e.annotate("index_0"))?;
        Ok(BootstrapEraDistr {
        })
    }
}

Current behavior

should not expose Fixed type in member, only needed for serializaiton: Fixed(Uint(1))

[rooooooooob]

Note: that error wasn't because you had bootstrapEraDistr = 1, it was because it was later used within a group choice as the only field, which is indeed a bug, but is a separate bug.

How do we know that the user will want to have an entire struct like that generated as well instead of just something like const BOOTSTRAP_ERA_DISTR: usize = 1?

[rooooooooob]

Note: the current behavior generates a wasm-exposed function like pub fn bootstrap_era_distr() -> u32 { 1 } (wasm doesn't support exposing constants) but I seem to have forgotten to make a special case for it on the rust side but could super easily generate a constant like the above. There would be more work for other people referring to it in other structs I think though.