Add GroupBy and GroupByMut iterators to the slice

[Kerollmops]

This PR propose to add two new methods to the slice, the group_by and group_by_mut. These two will provide a way to iterate over non-overlapping sub-slices of a base slice that are separated by the predicate given by the user (e.g. Partial::eq, |a, b| a.abs() < b.abs()).

An RFC has already been opened.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @sfackler (or someone else) soon.

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[rust-highfive]

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:04:58] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:58] tidy error: /checkout/src/libcore/slice/mod.rs:3963: line longer than 100 chars
[00:05:00] some tidy checks failed
[00:05:00] 
[00:05:00] 
[00:05:00] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:05:00] 
[00:05:00] 
[00:05:00] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:05:00] Build completed unsuccessfully in 0:01:48
[00:05:00] Build completed unsuccessfully in 0:01:48
[00:05:00] make: *** [tidy] Error 1
[00:05:00] Makefile:79: recipe for target 'tidy' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0f3df89c
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:0f682dde:start=1529235001557376032,finish=1529235001566097635,duration=8721603
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:00e62ed2
$ head -30 ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
head: cannot open ‘./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers’ for reading: No such file or directory
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:101bc604
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[rust-highfive]

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[01:18:05] ....................................................................................................
[01:18:20] ....................................................................................................
[01:18:36] ....................................................................................................
[01:18:50] ....................................................................................................
[01:19:10] ..........................F.........................................................................
[01:19:27] failures:
[01:19:27] 
[01:19:27] ---- slice/mod.rs - slice::[T]::group_by_mut (line 848) stdout ----
[01:19:27] error[E0308]: mismatched types
[01:19:27] error[E0308]: mismatched types
[01:19:27]   --> slice/mod.rs:855:1
[01:19:27]    |
[01:19:27] 10 | assert_eq!(iter.next(), Some(&[1, 1, 1][..]));
[01:19:27]    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
[01:19:27]    |
[01:19:27]    = note: expected type `std::option::Option<&mut [{integer}]>`
[01:19:27]               found type `std::option::Option<&[{integer}]>`
[01:19:27] 
[01:19:27] error[E0308]: mismatched types
[01:19:27]   --> slice/mod.rs:856:1
[01:19:27]    |
[01:19:27]    |
[01:19:27] 11 | assert_eq!(iter.next(), Some(&[3, 3][..]));
[01:19:27]    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
[01:19:27]    |
[01:19:27]    = note: expected type `std::option::Option<&mut [{integer}]>`
[01:19:27]               found type `std::option::Option<&[{integer}]>`
[01:19:27] 
[01:19:27] error[E0308]: mismatched types
[01:19:27]   --> slice/mod.rs:857:1
[01:19:27]    |
[01:19:27]    |
[01:19:27] 12 | assert_eq!(iter.next(), Some(&[2, 2, 2][..]));
[01:19:27]    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
[01:19:/obj/build/x86_64-unknown-linux-gnu/stage0-rustc/release
59904 ./obj/build/x86_64-unknown-linux-gnu/stage0-rustc/release/build
59488 ./obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps
56708 ./obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu
56704 ./obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[pietroalbini]

Closing this until the RFC is merged. When the RFC will be merged this will be reopened.