Add support for types that can be records or tuples

[KodrAus]

This PR adds support for "record tuples"; types that may be treated as either a record or a tuple by a Stream depending on its preference. The default preference is to treat these as records.

The motivation for this is to support types that can be streamed as tuples in binary formats like protobuf, or as records in human-readable formats like JSON, such as:

#[derive(sval::Value)]
pub struct Message<'a> {
    #[sval(index = 1)]
    pub id: i32,
    #[sval(index = 2, tag = sval_protobuf::tags::PACKED)]
    pub data: &'a [i32],
}

Alternatively, this same type could be defined as a tuple:

pub struct Message<'a>(
    #[sval(index = 1, label = "id")]
    i32,
    #[sval(label = "data", tag = sval_protobuf::tags::PACKED)]
    &'a [i32]
);

If a record has any fields with index specified then all fields must have it. Likewise for tuples and label.

[KodrAus]

I've been working on a protobuf implementation based on this just to sanity check everything. I think it all works out nicely, so I'll merge this one, but will wait until the protobuf support is complete before releasing.