Support bitflag enums (closes #158)

[lynn]

Now you can write

[flags]
enum MyFlags {
    Red = 1 << 0;
    Green = 1 << 1;
    Blue = 1 << 2;
    Yellow = Red | Green;
}

and this yields special codegen in Rust, specifically, where ordinary enums do not already allow bitflag-like behavior:

::bitflags::bitflags! {
    pub struct MyFlags: u32 {
        const RED = 1;
        const GREEN = 2;
        const BLUE = 4;
        const YELLOW = 3;
    }
}

At last, you can write let magenta = MyFlags::RED | MyFlags::BLUE and decode/encode values like that.

For now, I won't preserve the arithmetic expressions in codegen (sorry Andrew) and I won't hide the bitflags dependency behind a feature flag (sorry Matthew). Neither of those things seem worth the added complexity for right now.

Note to self: after this is merged, I need to go doc it on the wiki.

[mattiekat]

Why are these tokens being removed?

[lynn]

I changed the way attributes are parsed so that the "attribute name" after the opening [ is simply parsed as an identifier. This way we don't have to introduce a new keyword TokenKind for each new attribute that we want to introduce.

[mattiekat]

Maybe it would be good to add a const "isFlagsEnum: bool" to the enum definitions

Something like

export enum MyEnum {
 // ...
}

export const $MyEnumIsFlags = true;

Or maybe injecting it into the enum object and updating the typing to reflect? This seems a little harder though.

enum _MyEnum {...}
(_MyEnum as any).isFlags = true;
type MyEnumType = typeof _MyEnum & { isFlags: true }
export const MyEnum = _MyEnum as unknown as MyEnumType

I think that would work?