Simplify implementation of item equality

[complexspaces]

As discussed in #82, the current implementation seems non-ideal because it, intentionally or not, compares item paths and fetches their attributes at the same time once the paths are equal. Since all the fetching is done "just in time" and not from previously cached value on Item, these calls would always return the same attributes since the paths are also the same.

As this behavior has been in master for years now (seemingly without issue), it seems unlikely that anyone has run into the case where the path changes silently due to a provider change while an Item object is alive. If someone raises this issue later it can be evaluated then and fixed whenever the next major release of zbus comes out to accompany the breaking change required.

Additionally this PR takes advantage of the fact that the async Item implementation no longer needs to make calls via .await and implements PartialEq for the item instead to more closely match the blocking Item public API.

Closes #82