Generalize extend_with_drain to VecDeque and BinaryHeap.

[kpreid]

I also modified the code a little more than necessary, in order to make it clearer and shorter; in particular, src_ty is no longer shadowed and the immutable variable is gone.

changelog: [extend_with_drain]: now applies to uses of VecDeque and BinaryHeap as well as Vec

[rustbot]

r? @llogiq

rustbot has assigned @llogiq.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
• 7 candidates expanded to 7 candidates
• Random selection from Jarcho, dswij, llogiq, samueltardieu

[github-actions]

Lintcheck changes for 0f3eba5

Lint	Added	Removed	Changed
clippy::extend_with_drain	0	0	5

This comment will be updated if you push new changes