Add HasFlexibleArrayMember instance generation

[phadej]

AFAICT, everything else can be built as part of -runtime library on top, i.e. hs-bindgen generation part for flams is done.

[edsko]

@phadej We need a new class other than Storable no? And generate instances for that..? Or is that already done and I missed it?

[edsko]

Oh, no, wait, I get it. The "core" (without the flam) is Storable. The extended thing (newtype wrapped) is not, but as you said, we can do that in the lib. Ok, good.

[edsko]

@phadej Shall I rebase or do you prefer to do it?

[phadej]

We need a new class other than Storable no?

I don't think so. The support code for marshalling the whole struct (with flam) can be done without any additional generated code. HasFlexibleArrayMember and HasFlexibleArrayLength instances is enough; but are out of scope of this PR, which is about generation of HasFlexibleArrayMember.