Implement feature sort_unstable
Tracking issue for the feature: #40585
This is essentially integration of [pdqsort](https://github.com/stjepang/pdqsort) into libcore.
There's plenty of unsafe blocks to review. The heart of pdqsort is `fn partition_in_blocks` and is probably the most challenging function to understand. It requires some patience, but let me know if you find it too difficult - comments could always be improved.
#### Changes
* Added `sort_unstable` feature.
* Tweaked insertion sort constants for stable sort. Sorting integers is now up to 5% slower, but sorting big elements is much faster (in particular, `sort_large_big_random` is 35% faster). The old constants were highly optimized for sorting integers, so overall the configuration is more balanced now. A minor regression in case of integers is forgivable as we recently had performance improvements (#39538) that completely make up for it.
* Removed some uninteresting sort benchmarks.
* Added a new sort benchmark for string sorting.
#### Benchmarks
The following table compares stable and unstable sorting:
```
name stable ns/iter unstable ns/iter diff ns/iter diff %
slice::sort_large_ascending 7,240 (11049 MB/s) 7,380 (10840 MB/s) 140 1.93%
slice::sort_large_big_random 1,454,138 (880 MB/s) 910,269 (1406 MB/s) -543,869 -37.40%
slice::sort_large_descending 13,450 (5947 MB/s) 10,895 (7342 MB/s) -2,555 -19.00%
slice::sort_large_mostly_ascending 204,041 (392 MB/s) 88,639 (902 MB/s) -115,402 -56.56%
slice::sort_large_mostly_descending 217,109 (368 MB/s) 99,009 (808 MB/s) -118,100 -54.40%
slice::sort_large_random 477,257 (167 MB/s) 346,028 (231 MB/s) -131,229 -27.50%
slice::sort_large_random_expensive 21,670,537 (3 MB/s) 22,710,238 (3 MB/s) 1,039,701 4.80%
slice::sort_large_strings 6,284,499 (38 MB/s) 6,410,896 (37 MB/s) 126,397 2.01%
slice::sort_medium_random 3,515 (227 MB/s) 3,327 (240 MB/s) -188 -5.35%
slice::sort_small_ascending 42 (1904 MB/s) 41 (1951 MB/s) -1 -2.38%
slice::sort_small_big_random 503 (2544 MB/s) 514 (2490 MB/s) 11 2.19%
slice::sort_small_descending 72 (1111 MB/s) 69 (1159 MB/s) -3 -4.17%
slice::sort_small_random 369 (216 MB/s) 367 (217 MB/s) -2 -0.54%
```
Interesting cases:
* Expensive comparison function and string sorting - it's a really close race, but timsort performs a slightly smaller number of comparisons. This is a natural difference of bottom-up merging versus top-down partitioning.
* `large_descending` - unstable sort is faster, but both sorts should have equivalent performance. Both just check whether the slice is descending and if so, they reverse it. I blame LLVM for the discrepancy.
r? @alexcrichton
travis: Don't set `RUST_LOG` globally
I have a suspicion that this caused a large regression in cycle times by forcing
the compiler to perform more checks on every `debug!` statement, so let's test
this out by removing the `RUST_LOG` env var globally.
This regression in cycle time was witnessed between [two] [builds] where the
[PR] in question didn't do much suspicious. Judging by how the stage0 times
*also* regressed though then this is my best guess.
[two]: https://travis-ci.org/rust-lang/rust/builds/210149932
[builds]: https://travis-ci.org/rust-lang/rust/builds/210179995
[PR]: https://github.com/rust-lang/rust/pull/40446
fix innacuracy in mir TerminatorKind::SwitchInt docs
Each index of `values` corresponds to an index of `targets`, and `targets` additionally has a "default case" element at its end, so `targets.len() == values.len() + 1`, not the other way around. For example, [here](0aeb9c1297/src/librustc/mir/mod.rs (L549-L550)) is a concrete instance of `SwitchInt` being constructed with `targets.len() == 2` and `values.len() == 1`.
Extract book into a submodule
Part of https://github.com/rust-lang/rust/issues/39588
We probably don't want to land this till after the beta branches on friday, but would still ❤️ a review from @alexcrichton , since I am a rustbuild noob.
This pr:
1. removes the book
2. adds it back in as a submodule
3. the submodule includes both the old book and the new book
4. it also includes an index page explaining the difference in editions
5. it also includes redirect pages for the old book URLs.
6. so we build all that stuff too.
r? @alexcrichton
