Add an annotation for deriving MallocSizeOf that uses MallocUnconditionalSizeOf for specific fields

[jdm]

Whether it's derive(MallocSizeOf(unconditional = "field1,field2")) on a type or #[unconditional_malloc_size_of] on a particular field, this would make it much easier to deal with structures that contain a mixture of fields that can be automatically derived and fields that cannot (eg. #20391).

[highfive]

cc @fitzgen