Accessing fields of substructs

[Hejsil]

Currently, this does not work:

const SubOptions = struct {
    b: u64,
};

const Options = struct {
    a: SubOptions,
};
const parser = comptime Clap(Options).Builder
    .init(
        Options {
            .a = SubOptions {
                .b = 0
            }
        }
    )
    .command(
        Command.Builder
            .init("command")
            .arguments(
                []Argument {
                    Argument.Builder
                        .init("a.b")
                        .help("Set the 'b' field of Option.")
                        .short('b')
                        .takesValue(true)
                        .build(),
                }
            )
            .build()
    )
    .build();

zig-clap/clap.zig:193:13: error: Field not found!
            @compileError("Field not found!");
            ^
zig-clap/clap.zig:196:89: note: called from here
        fn getFieldPtr(comptime T: type, res: &T, comptime field: []const u8) &FieldType(T, field) {
                                                                                        ^
zig-clap/clap.zig:196:79: note: called from here
        fn getFieldPtr(comptime T: type, res: &T, comptime field: []const u8) &FieldType(T, field) {
                                                                              ^
zig-clap/clap.zig:144:111: error: unable to evaluate constant expression
                                        *getFieldPtr(Result, &result, option.field) = try strToValue(FieldType(Result, option.field), value);
                                                                                                              ^
zig-clap/clap.zig:23:32: note: called from here
            return parseCommand(CommandList { .command = clap.command, .prev = null }, clap.defaults, arguments);
                               ^
zig-clap/sample.zig:49:37: note: called from here
    const options = try parser.parse(args[1..]);

Also, the error message should probably be better.