Expose number of variants when serializing enum

[dtolnay]

From IRC:

<cmr> Hm, this seems problematic. My serializer can't keep unbounded state, but I want to encode enum variant tags with the minimum amount of bytes necessary, using the same representation Rust does ideally.
<cmr> The obvious approach of using a stack of recent total-number-of-variants is out, but I don't see how else it's possible.
<cmr> (in fact, afaict, the serializer *never even has access* to that information, although the deserializer does.)
<cmr> in the form of the len of the variants arg to deserialize_enum

cc @cmr

[dtolnay]

@cmr I don't have the IRC logs but if I remember correctly, you realized that aiming for an encoding that is "at most as many bytes as Rust" was not feasible because of NonZero-related optimizations such as for Option<&T>, not to mention rust-lang/rfcs#1230. For just minimizing number of bytes, a variable-length encoding seems like it would be a better approach. The vast majority of enums will use 1 byte.

I am inclined to not make this change, rather than complicating the Serializer interface for this use case. What do you think?