New/improved warning option for padding bytes

[Jonas Rahlf]

As a firmware developer I would appreciate a warning option about padding
bytes, but only for such cases where you can reduce the number of padding
bytes by rearranging the fields.

The current -Wpadded warns about all padding bytes, which is annoying.

The concept sounds simple: you rearrange all fields from large to small and
check if the size changed.

The goal is to more easily optimize RAM/flash in space constrained systems.

Jonas