enum tweaks

[bernardnormier]

In light of #30 (comment), I propose to make a few tweaks to enums.

1. An enum defined in an encoding = 2 file must have an underlying type.
   Rationale: we want to reserve the Slice2 encoding of enums (both checked and unchecked) without underlying type for the future enums-with-values.

2. A checked or unchecked enum defined in an encoding = 1 file cannot have an underlying type (that's already the case today).
   Rationale: there is only one Slice1 encoding for enums and it would be confusing the add an optional underlying type with no effect on the Slice1 encoding.

3. A checked or unchecked enum defined in an encoding = 1 file is mapped and encoded with Slice2 as if if had been defined in an encoding = 2 file with the varint32 underlying type. (Already the case today).

Rationale: language-mapping wise, that's the only reasonable behavior.

This means:

encoding = 1
unchecked enum Error { NotFound }

cannot carry "-1" when encoded with Slice1, but can when encoded with Slice2.

[InsertCreativityHere]

This was implemented in #489.