[API] Add FFI wrapper modifier to strong typedefs

[emesare]

Apart of https://github.com/Vector35/binaryninja/pull/1735 (Binary Similarity)
For a test_* build with this change see the above branch ^

This modifier is used for wrapping and unwrapping FFI structures representing the raw value type.

In particular we are using it in the binary similarity like:

namespace BinaryNinjaCore {
	namespace st = ::bn::base::strong_typedef;
	using SimilarityEntityId = ::bn::base::StrongTypedef<uint32_t, struct SimilarityEntityIdTag,
		st::FfiWrapper<BNSimilarityEntityId, &BNSimilarityEntityId::value>, st::Ordered, st::Hashable,
		st::Incrementable>;
}  // namespace BinaryNinjaCore

Where we want to use the strong_typedef modifiers to derive functionality for use in containers (hashable, ordered) and still preserve the relationship between the C++ and C (via FfiWrapper).

Also see https://github.com/Vector35/binaryninja/pull/1737 for unit tests.

[bdash]

I really like the idea, but there are a couple of changes that would help this fit better with our existing C++ <-> C API conversions.

[emesare]

Ah yes, the initializer change really does improve the usability, made the requested changes, thanks!