Use for_each to extend collections

[cuviper]

This updates the Extend implementations to use for_each for many
collections: BinaryHeap, BTreeMap, BTreeSet, LinkedList, Path,
TokenStream, VecDeque, and Wtf8Buf.

Folding with for_each enables better performance than a for-loop for
some iterators, especially if they can just forward to internal
iterators, like Chain and FlatMap do.

[rust-highfive]

r? @TimNN

(rust_highfive has picked a reviewer for you, use r? to override)

[cuviper]

For reference:

• Vec was updated in Make spec_extend use for_each() #54761
• String was updated in Optimized string FromIterator + Extend impls #56548
• HashMap is being replaced by hashbrown in Replace HashMap implementation with SwissTable (as an external crate) #58623
  • The same treatment is offered in Amanieu/hashbrown#58

[Centril]

r? @scottmcm

[Mark-Simulacrum]

To get at least a sense that this isn't obviously bad (though I don't expect it to be), let's @bors try

[bors]

⌛ Trying commit 0730a01 with merge eab4756...

[scottmcm]

r=me when perf is happy

[scottmcm]

Actually, you might check for while let loops too:

rust/src/liballoc/vec.rs

Line 1929 in 9ebf478

while let Some(element) = iterator.next() {

[cuviper]

I see, the previous Vec improvement was only for I: TrustedLen. I'll look for more...

[cuviper]

@scottmcm that one is difficult, because it wants to check size_hint() every time it needs to grow, which we can't do while folding...

[bors]

☀️ Try build successful - checks-travis
Build commit: eab4756

[cuviper]

@rust-timer build eab4756

[rust-timer]

Success: Queued eab4756 with parent acd8dd6, comparison URL.

[rust-timer]

Finished benchmarking try commit eab4756

[scottmcm]

@cuviper Oops! You're right. And perf looks happy, so

@bors r+ rollup

[bors]

📌 Commit 0730a01 has been approved by scottmcm