Add function core::iter::chain

[rossmacarthur]

The addition of core::iter::zip (#82917) set a precedent for adding plain functions for iterator adaptors. Adding chain makes it a little easier to chain two iterators.

for (x, y) in chain(xs, ys) {}
// vs.
for (x, y) in xs.into_iter().chain(ys) {}

There is prior art for the utility of this in itertools::chain.

Pending ACP rust-lang/libs-team#154

[rustbot]

r? @joshtriplett

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
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[rossmacarthur]

@rustbot label +T-libs-api -T-libs

[Nilstrieb]

You need to create an ACP first.

[pitaj]

@rustbot label -S-waiting-on-author +S-waiting-on-ACP

[joshtriplett]

r? libs

[joshtriplett]

Correction:

r? libs-api