Implement std::error::Error for CollectionAllocErr

[JarvisCraft]

Description

It seems unnatural that CollectionAllocErr currently does not implement std::error::Error which would come in handy in cases such as implementing std::error::Error::source on user-defined errors caused by CollectionAllocErr.

While error_in_core is not stable yet, such implementation could be hidden behind a feature flag such as (commmonly used) std.

Since the features should be additive (i.e. an inverted no-std feature would not be the right approach), it is a breaking change to add such a feature so I suggest adding this to #183.

If this change is welcome, I may create a pull request to implement it.

cc @mbrubeck as the maintainer of the 2.0 wishlist

[mbrubeck]

Yes, a pull request to the v2 branch would be welcome.

I don't think this is a breaking change. Adding a disabled-by-default std feature should have no effect on existing users of the crate. However, I prefer not to make any unnecessary changes to v1 at this point. Also, if we do this on v2 then we can also deprecate the write feature in favor of the new std feature.