Enforce SP ABI standard layout

[mrwonko]

The SP ABI currently contains non-standard layout types (e.g. std::vector) which differ across compilers/compiler versions. Can't have those if we want SP code mods.

Types can be checked at compile time using std::is_standard_layout.

(Is there already an issue about this? We've certainly talked about it previously, but I can't remember where.)

[ensiform]

Where is it used in the ABI?

[mrwonko]

CGhoul2Info_v parameter in G2API_SetSkin() & co, at the very least.