basic chapter for representation of unions

[RalfJung]

Fixes #13

[nikomatsakis]

@RalfJung maybe it's worth noting that #[repr(C)] is probably also needed for ABI compatibility when invoking functions (although at present I expect we are de facto compatible).

[RalfJung]

maybe it's worth noting that #[repr(C)] is probably also needed for ABI compatibility when invoking functions (although at present I expect we are de facto compatible).

Not sure what you mean. Are you just saying "to pass unions by value to C, they need to be repr(C)"? But isn't that true for anything? Might still be worth repeating, as long as we make sure it is also state in every FFI guide as well as in the struct and enum chapters.