Fix encoding of packed variants in new enum format

[matix2267]

Fixes #187

Legacy enum format supported packed encoding pretty well but with the
switch to new enum format there was a regression (#173).
This commit fixes the new enum format in packed encoding.

For example consider the following structure:

enum Enum {
    Unit,
    NewType(i32),
    Tuple(i32, i32),
    Struct { x: i32, y: i32 },
}

Legacy enum packed encodings are:

Empty: <variant number>
NewType: [<variant number>, value]
Tuple: [<variant number>, values..]
Struct: [<variant number>, {<struct>}]

Non-legacy enum packed encodings before this commit:

Empty: <variant number>
NewType: {"<variant>": value}
Tuple: {"<variant>": [values..]}
Struct: {<variant number>: {<struct>}}

Notice how NewType and Tuple store the name instead of variant number.

Fixed after this commit:

Empty: <variant number>
NewType: {<variant number>: value}
Tuple: {<variant number>: [values..]}
Struct: {<variant number>: {<struct>}}

After this commit the packed encoding can be briefly described as:
All struct fields and enum variants are encoded as their field number
rather than name.
This applies to all types of members (unit, newtype, tuple and struct).