Add a public getFilling function

[dpwiz]

I would like to have a structure preview capability. This is a stepping stone towards it.

> data Foo = Foo Word8 Float Word32 Float deriving (Show, Generic, GStorable)
> getFilling (Foo 0 0 0 0)
[Size 1,Padding 3,Size 4,Size 4,Size 4]

[dpwiz]

Hm... The git master appears to be older than Hackage code 🤔

[mkloczko]

Yeah, sorry, forgot to push changes ^^'.

[dpwiz]

Thanks. I've updated the PR with docs and a provisional version bump.

[mkloczko]

Thanks, will rename the function and the Filling data type, probably into gmemoryLayout or something like that.

[dpwiz]

Would be cool if the sizes could be annotated with field and type names 🤔
That'll make generating actual C structs almost trivial.