Add packed struct encoding

[Nirlah]

A NS_OPTIONS macro can be represented in Zig using a packed struct.
This PR add support for implicitly encoding such options structs as integers for use in blocks and methods.

For example, the following type is mapped into a packed struct in Zig.
It can than be used trivially as a method parameter.

typedef NS_OPTIONS(NSUInteger, NSApplicationPresentationOptions) {
    NSApplicationPresentationDefault = 0,
    NSApplicationPresentationAutoHideDock = (1 <<  0),
    // ...
    NSApplicationPresentationDisableCursorLocationAssistance API_AVAILABLE(macos(10.11.2)) = (1 << 12)
}

pub const NSApplicationPresentationOptions = packed struct(c_ulonglong) {
    auto_hide_dock: bool = false,
    // ...
    disable_cursor_location_assistance: bool = false,
    _: u51 = 0,

    pub const default: @This() = .{};
};