Extensions has over restrictive bounds.

[carloskiki]

Inserting and removing from a Extensions map should not require Clone or Sync. We never access a reference to the stored value, and we never clone it, hence I do not see why Extensions requires these bounds.

I came across this issue as I wanted to store a oneshot::Sender that is not Clone nor Sync for good reasons.

[sfackler]

Extensions erases the types inside of it. If it did not require all types to be Send and Clone, it could not be Send or Clone itself. See #395 for why the Clone bound was added in 1.0.