decode

    pub fn decode(bytes: []const u8, index: Index) DecodeError!Element {
        if (index > bytes.len) return error.EndOfStream;
        var reader: std.Io.Reader = .fixed(bytes[index..]);

        const tag = Tag.decode(&reader) catch |err| switch (err) {
            error.ReadFailed => unreachable, // it's all fixed buffers
            else => |e| return e,
        };
        const size_or_len_size = reader.takeByte() catch |err| switch (err) {
            error.ReadFailed => unreachable, // it's all fixed buffers
            else => |e| return e,
        };

        const len = if (size_or_len_size < 128)
            // short form between 0-127
            size_or_len_size
        else blk: {
            // long form between 0 and std.math.maxInt(u1024)
            const len_size: u7 = @truncate(size_or_len_size);
            if (len_size > @sizeOf(Index)) return error.EndOfStream;

            const len = reader.takeVarInt(Index, .big, len_size) catch |err| switch (err) {
                error.ReadFailed => unreachable, // it's all fixed buffers
                else => |e| return e,
            };
            if (len < 128) return error.EndOfStream; // should have used short form

            break :blk len;
        };

        const start = index + @as(Index, @intCast(reader.seek));
        const end = std.math.add(Index, start, len) catch return error.EndOfStream;
        if (end > bytes.len) return error.EndOfStream;

        return Element{ .tag = tag, .slice = Slice{ .start = start, .end = end } };
    }