Use enum in codegen to specify constant-constants

[jleibs]

Our enums currently get converted into classes that implement arrow unions, which I don't believe is correct. The more common pattern of using enums in fbs files, for example:

enum ViewDir: byte {
    Up = 1,
    Down = 2,
    Right = 3,
    Left = 4,
    Forward = 5,
    Back = 6,
}

Should map to an appropriate enum of constants in each language.

[Wumpf]

would also be useful for log level strings!

[abey79]

...and for blueprint container kind.

[emilk]

There are two ways we can encode the enums: using names ("Up", "Down", …), or using integers (1, 2, …).

I think using the names is superior, as it means the arrow data is self-descriptive.
This means viewing them in e.g. the table view would show the variant name, instead of just an integer.
However, it means renaming an enum variant is a breaking change (just like for the name of a struct field).

I believe the best way to encode these in Arrow is by using a Sparse Union with the NULL type for each field. This means only 8 bits should be required for each value.