Add derive macro for SpreadAllocate trait

[Robbepop]

#978 has added a new SpreadAllocate trait which is useful for default initializing ink! contracts onto some storage region which was not really possible before.

This trait is very much needed for non-caching storage facilities and collections such as the Mapping type first introduced in #946 and refined in #979.

However, currently we are missing a simple way for users to derive the trait.
This issue is about implementing a derive macro for the new SpreadAllocate trait.
The where clause should make sure that it is only implemented on types where all fields are SpreadAllocate, too.

For this the ink_storage_derive crate has to be adjusted.

[HCastano]

Closed by #995.