feat(chain): export type alias for FutureResult

[rustaceanrob]

Description

Similar to BoxedFuture it may be helpful for users to use this alias when implementing PersistAsyncWith

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[oleonardolima]

utACK 8ee6c02

[evanlinjin]

ConceptACK.

However, given that #1547 will go into beta (based on my understanding), it might be better to base this on that! I'm wondering the same thing for #1552

[rustaceanrob]

Rebased, although I did not review the recent PRs so I am not sure if this is still helpful.

[evanlinjin]

I'm thinking now, that the better approach would be to depend on futures-core = { version = "0.3" }.

[rustaceanrob]

futures_core contains a FutureResult type. Closing this