update stdsimd dependency

[hellow554]

Update to at latest commit (00f88e7) to silence this warning:

warning: allow_internal_unstable expects list of feature names. In the future this will become a hard error. Please use allow_internal_unstable(foo, bar) to only allow the foo and bar features

Fixes rust-lang/stdarch#681

[rust-highfive]

r? @alexcrichton

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[rust-highfive]

⚠️ Warning ⚠️

• These commits modify submodules.

[rust-highfive]

The job x86_64-gnu-llvm-6.0 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.

travis_time:end:020e107c:start=1550134608713528647,finish=1550134609681685895,duration=968157248
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---
[00:06:14]    Compiling cc v1.0.28
[00:06:14]    Compiling core v0.0.0 (/checkout/src/libcore)
[00:06:14]    Compiling libc v0.2.46
[00:06:14]    Compiling unwind v0.0.0 (/checkout/src/libunwind)
[00:06:15] error[E0658]: #[doc(include = "...")] is experimental (see issue #44732)
[00:06:15]    |
[00:06:15]    |
[00:06:15] 10 |     doc(include = "../stdsimd/crates/core_arch/src/core_arch_docs.md")
[00:06:15]    |
[00:06:15]    = help: add #![feature(external_doc)] to the crate attributes to enable
[00:06:15] 
[00:06:15]    Compiling build_helper v0.1.0 (/checkout/src/build_helper)
---
[00:06:33] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "--message-format" "json"
[00:06:33] expected success, got: exit code: 101
[00:06:33] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap build
[00:06:33] Build completed unsuccessfully in 0:00:21
[00:06:33] make: *** [all] Error 1
[00:06:33] Makefile:18: recipe for target 'all' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0b6e98ee
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Thu Feb 14 09:03:35 UTC 2019
---
travis_time:end:1c9ef4c2:start=1550135016041353710,finish=1550135016046187806,duration=4834096
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:06cb6ea7
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:04fbdb78
travis_time:start:04fbdb78
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:0659b392
$ 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)

[hellow554]

Interesting. https://github.com/rust-lang-nursery/stdsimd/blob/00f88e7f2d7ec8ee9090514a03f3def3c106227c/crates/core_arch/src/lib.rs#L35 defines #![feature(external_doc)]. I guess I don't have enough knowledge on compiler internals to understand what's going on here.

[alexcrichton]

Thanks for this! As you've seen it's unfortunately a bit tricky on this update, but it's also being handled at #58373 so I'm gonna close this.