feat(enum): allow for enum tag discriminant to be u16

Current spec says:

```
enums are serialized with using u8 for the enum ordinal 
```

Can we make it support u16 too?

Current experimental `borsh-rs` schema  allows tag_width to be 2 bytes, which allows to discriminant to be u16.

I tried to expand u8 to 16, but I did not even know there was spec stating u8 (https://github.com/near/borsh-rs/pull/315)