futures-core 0.3.7 is breaking

[leo60228]

The following chain of events can lead to futures-util failing to build despite none of the steps being individually abnormal:

1. Add futures-util 0.3.6 as a dependency.
2. Later, add a dependency depending on a newer version of futures-core.

Here's a repo demonstrating this (check the commit history): https://github.com/leo60228/futures-test

error[E0432]: unresolved import `futures_core::core_reexport`
  --> /home/leo60228/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/lib.rs:55:9
   |
55 | pub use futures_core::core_reexport;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `core_reexport` in the root

This is because futures-core 0.3.7 removed core_reexport. This seems like it should be fine, since it's only used internally by futures, but mismatched subcrate versions can cause this issue.

[taiki-e]

Fixed by #2233 (0.3.7) that removing the dependency on private API.