Enhance Pin impl applicability for PartialEq and PartialOrd.

[pthariensflame]

This allows for comparing for equality or ordering a Pin<P> and a Pin<Q> as long as P and Q are correspondingly comparable themselves even when P and Q are different types.
An example might be comparing a Pin<&mut OsString> to a Pin<&mut PathBuf>, which might arise from pin projections from a pair of larger contexts that aren't Unpin.

[Centril]

r? @withoutboats

cc @cramertj

[withoutboats]

@bors r+

[bors]

📌 Commit 22251a8 has been approved by withoutboats

[Centril]

@bors rollup

[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:0066eee4:start=1547692080936907462,finish=1547692082378653094,duration=1441745632
$ 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
---

[00:03:32] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:03:32] tidy error: /checkout/src/libcore/pin.rs:132: trailing whitespace
[00:03:32] tidy error: /checkout/src/libcore/pin.rs:147: trailing whitespace
[00:03:34] some tidy checks failed
[00:03:34] 
[00:03:34] 
[00:03:34] 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"
[00:03:34] 
[00:03:34] 
[00:03:34] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:34] Build completed unsuccessfully in 0:00:47
[00:03:34] Build completed unsuccessfully in 0:00:47
[00:03:34] Makefile:69: recipe for target 'tidy' failed
[00:03:34] make: *** [tidy] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:094e3674
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Thu Jan 17 02:31:48 UTC 2019
---
travis_time:end:2155442a:start=1547692308763340948,finish=1547692308767892582,duration=4551634
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:021f97ac
$ 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:036b24ff
travis_time:start:036b24ff
$ 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:1012a880
$ 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)

[Centril]

remove this line

[Centril]

and here too.

[Centril]

@bors r-

tidy errors

[pthariensflame]

@Centril Fixed! Sorry about that; I can't run tidy on my local machine for some reason.

[Centril]

@bors r=withoutboats rollup

[bors]

📌 Commit fefe1da has been approved by withoutboats