Implement better .is_empty() for slice and vec iterators

[bluss]

These iterators can use a pointer comparison instead of computing the length.

[rust-highfive]

r? @sfackler

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

[bluss]

The answer to “doesn't the compiler figure this out” is no, see playpen https://is.gd/o09MRd

[alexcrichton]

@bors: r+

[bors]

📌 Commit f6becc9 has been approved by alexcrichton

[frewsxcv]

@bors r-

error: use of unstable library feature 'exact_size_is_empty' (see issue #35428)
   --> /checkout/src/libcollections/../libcollectionstest/slice.rs:641:40
    |
641 |             assert_eq!(xs[i..j].iter().is_empty(), xs[i..j].is_empty());
    |                                        ^^^^^^^^
    |
    = help: add #![feature(exact_size_is_empty)] to the crate attributes to enable

error: variable does not need to be mutable
   --> /checkout/src/libcollections/../libcollectionstest/slice.rs:638:9
    |
638 |     let mut xs = [1, 2, 5, 10, 11];
    |         ^^^^^^
    |
note: lint level defined here
   --> /checkout/src/libcollections/../libcollectionstest/lib.rs:11:9
    |
11  | #![deny(warnings)]
    |         ^^^^^^^^

error: aborting due to 2 previous errors

Build failed, waiting for other jobs to finish...
error: Could not compile `collections`.

[frewsxcv]

@bors: r-

[bluss]

Thanks, I fixed the test by amending

@bors r=alexcrichton

[bors]

📌 Commit 74cde12 has been approved by alexcrichton

[bors]

⌛ Testing commit 74cde12 with merge 217f57c...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: alexcrichton
Pushing 217f57c to master...