Enumerations

[brendanzab]

Add some form of enumerations to the DDL. These are a set of grouped constants that may or may not be used for flags.

[brendanzab]

Thinking we could have an enumeration type like so:

ByteOrder : Type = enum {
    little,
    big,
};

This is sort of like a 'labelled' syntax over 'finite sets', as I tracked down in this twitter thread.

We'd also have a corresponding Format constructor:

ByteOrderFormat : Format = format enum U8 {
    little = 0x4949,
    big = 0x4d4d,
};

Where repr ByteOrderFormat is equal to ByteOrder

We could use this for defining unions:

Exif = format struct {
    byte_order : ByteOrderFormat,
    body : match byte_order {
        little => ...,
        big => ...,
    },
};

This would nicely solve both #15 and #118 as well.