Handle implied padding in composite types and at end of blocks

[threewholefish]

There was some code to handle implied padding, but it didn't quite go far enough for some functions.

The first type of padding is between composite type components, as indicated by the offset attribute of said components. The second is padding at the end of blocks to make the total length equal to the stated blockLength This happens for message fields (groups do not count towards the message blockLength) and group fields.