Consider adding support for packed field layout

[densh]

Currently fields are layed out sequentially and padded as required by the data types they are holding. It would be great to also support packed layout where user tells the library that order of the fields doesn't matter and that they can be reordered to decrease the size. E.g.

@data class C(x: Short, y: Long, x: Short)                // 24 bytes 
@data(packed = true) class C(x: Short, y: Long, z: Short) // 16 bytes

[densh]

Not enough interest.