Allow for more than 256 variant enum: encode variant with compact encoding for integer.

[thiolliere]

As proposed by Gav: we should allow for enum with more than 256 variants, and we can do encoding of the variant index using compact encoding of integers. This way this is not breaking for enum with 0-63 variants.

For the transition we can follow this:

• having 2 new attributes when deriving encode/decode for enum:
  • compact_variant_index (or a better name)
  • u8_variant_index
• then we can deprecate the derivation of encode/decode on enum without any attribute
• then we can force to use one of the 2 attributes
• then we can make the default being compact_variant_index.