Added StaticDataSize trait and derive macros for DataSize and StaticDataSize

[Syudagye]

It is now possible to implement StaticDataSize and DataSize traits with derive macro !

// Both enums and structs with named or unnamed fields are supported

// Deriving for StaticDataSize will also implement DataSize
// You don't need to add a derive for DataSize (It will not compile anyways)
#[derive(StaticDataSize)] // The size is known at compile time
enum TestSizedEnum {
	Unit,
	Unnamed(u16),
	Named { field1: u32, field2: i8 },
}
#[derive(DataSize)] // The size will change
struct TestDynamicTuple(Vec<Option<u64>>, i64);

// Even Generics works !!!
#[derive(DataSize, StaticDataSize)] // Here T could be statically sized or not, so we can derive both traits
struct TestStructGenerics<'a, T> {
	value: &'a T,
	wrapper: Option<T>,
	enum_value: TestEnumGenerics<Option<T>>,
}

The only thing left to do is the documentation :)

[Antikyth]

This is good! There are some things that I think need to be changed (see comments), and some that could be changed but mostly just for readability :)

[Antikyth]

I didn't know about the specialization feature - that seems really useful, especially for this kinda thing! Is it a concern that you have to even allow a lint for incomplete_features on top of enabling the feature flag though? Sounds like this feature is likely to change or possibly even be removed?

[Syudagye]

I didn't know about the specialization feature - that seems really useful, especially for this kinda thing! Is it a concern that you have to even allow a lint for incomplete_features on top of enabling the feature flag though? Sounds like this feature is likely to change or possibly even be removed?

Yes that is really useful for this right now.
But yes it is not complete, there is an issue with "soundness" if i remember well:
https://rust-lang.github.io/rfcs/1210-impl-specialization.html
rust-lang/rust#31844
rust-lang/rfcs#1210

[Antikyth]

By the way, just a terminology thing: lifetimes are generics. In the same way that generic types represent a non-specific type with certain bounds (such as the places that generic type is put), lifetimes are generic in that they don't represent a specific length of time, rather just a non-specific length of time, where you may annotate that certain things live for at least the same lifetime as others by placing lifetimes in those positions. So 'generics' refers to both generic types and lifetimes :)

[Antikyth]

Not done reviewing, will finish probably tomorrow. Looks good! Maybe one or two minor things I guess, too tired to read properly now :p

[Antikyth]

I realize this should be 1usize right? To include the discriminant?