skipPadding

    fn skipPadding(
        encoded: []const u8,
        padding_len: usize,
        ignored_chars: ?StaticBitSet(256),
    ) error{InvalidPadding}![]const u8 {
        var b64_pos: usize = 0;
        var i = padding_len;
        while (i > 0) {
            if (b64_pos >= encoded.len) {
                return error.InvalidPadding;
            }
            const c = encoded[b64_pos];
            if (c == '=') {
                i -= 1;
            } else if (ignored_chars) |set| {
                if (!set.isSet(c)) {
                    return error.InvalidPadding;
                }
            }
            b64_pos += 1;
        }
        return encoded[b64_pos..];
    }