Rollup merge of #119468 - notriddle:notriddle/compression, r=GuillaumeGomez

rustdoc-search: tighter encoding for f index

Depends on https://github.com/rust-lang/rust/pull/119457

Two optimizations for the function signature search:

* Instead of using JSON arrays, like `[1,20]`, it uses VLQ
  hex with no commas, like `[aAd]`.
* This also adds backrefs: if you have more than one function
  with exactly the same signature, it'll not only store it once,
  it'll *decode* it once, and store in the typeIdMap only once.

Based partially on discussions on zulip:
https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/search.20index.20size

Performance
-----------

https://notriddle.com/rustdoc-html-demo-8/compression-perf-v2/index.html

### memory/time profiler output (for more details, consult the above link)

<table>
<thead><tr><th>benchmark<th>before<th>after</tr></thead>
<tbody>
<tr><th>arti<td>

```
user: 002.789 s
sys:  000.390 s
wall: 002.096 s
child_RSS_high:     440796 KiB
group_mem_high:     414924 KiB
```

</td><td>

```
user: 002.295 s
sys:  000.278 s
wall: 001.738 s
child_RSS_high:     314588 KiB
group_mem_high:     285220 KiB
```

</td></tr><tr><th>cortex-m<td>

```
user: 000.127 s
sys:  000.030 s
wall: 000.134 s
child_RSS_high:      60264 KiB
group_mem_high:      23824 KiB
```

</td><td>

```
user: 000.136 s
sys:  000.038 s
wall: 000.137 s
child_RSS_high:      59204 KiB
group_mem_high:      22712 KiB
```

</td></tr><tr><th>sqlx<td>

```
user: 000.887 s
sys:  000.118 s
wall: 000.592 s
child_RSS_high:     190408 KiB
group_mem_high:     157804 KiB
```

</td><td>

```
user: 000.798 s
sys:  000.101 s
wall: 000.525 s
child_RSS_high:     159292 KiB
group_mem_high:     126292 KiB
```

</td></tr><tr><th>stm32f4<td>

```
user: 013.884 s
sys:  005.399 s
wall: 013.149 s
child_RSS_high:    1942244 KiB
group_mem_high:    1954916 KiB
```

</td><td>

```
user: 006.128 s
sys:  003.297 s
wall: 007.994 s
child_RSS_high:    1038108 KiB
group_mem_high:    1023900 KiB
```

</td></tr><tr><th>ripgrep<td>

```
user: 000.441 s
sys:  000.063 s
wall: 000.264 s
child_RSS_high:     109180 KiB
group_mem_high:      74272 KiB
```

</td><td>

```
user: 000.408 s
sys:  000.044 s
wall: 000.238 s
child_RSS_high:     101488 KiB
group_mem_high:      66000 KiB
```

</td></tr></tbody></table>

Size change
-----------

standard library without gzip:

```console
$ du -bs search-index-old.js search-index-new.js
4976370 search-index-old.js
4404391 search-index-new.js
```

((4976370-4404391)/4404391)*100% = 12.9%

with gzip:

```console
$ du -hs search-index-old.js.gz search-index-new.js.gz
520K    search-index-old.js.gz
504K    search-index-new.js.gz

$ du -bs search-index-old.js.gz search-index-new.js.gz
522092  search-index-old.js.gz
507654  search-index-new.js.gz
```

((522092-507654)/507654)*100% = 2.8%

Benchmarks are similarly shrunk.

Without gzip:

```console
$ du -hs tmp/{arti,cortex-m,sqlx,stm32f4,ripgrep}/toolchain_{old,new}/doc/search-index.js
10555067        tmp/arti/toolchain_old/doc/search-index.js
8921236 tmp/arti/toolchain_new/doc/search-index.js
77018   tmp/cortex-m/toolchain_old/doc/search-index.js
66676   tmp/cortex-m/toolchain_new/doc/search-index.js
2876330 tmp/sqlx/toolchain_old/doc/search-index.js
2436812 tmp/sqlx/toolchain_new/doc/search-index.js
63632890        tmp/stm32f4/toolchain_old/doc/search-index.js
52337438        tmp/stm32f4/toolchain_new/doc/search-index.js
631150  tmp/ripgrep/toolchain_old/doc/search-index.js
541646  tmp/ripgrep/toolchain_new/doc/search-index.js
```

With gzip:

```console
$ du -bs tmp/{arti,cortex-m,sqlx,stm32f4,ripgrep}/toolchain_{old,new}/doc/search-index.js.gz
1618852 tmp/arti/toolchain_old/doc/search-index.js.gz
1582007 tmp/arti/toolchain_new/doc/search-index.js.gz
16109   tmp/cortex-m/toolchain_old/doc/search-index.js.gz
15831   tmp/cortex-m/toolchain_new/doc/search-index.js.gz
422257  tmp/sqlx/toolchain_old/doc/search-index.js.gz
411507  tmp/sqlx/toolchain_new/doc/search-index.js.gz
4454761 tmp/stm32f4/toolchain_old/doc/search-index.js.gz
4334924 tmp/stm32f4/toolchain_new/doc/search-index.js.gz
98312   tmp/ripgrep/toolchain_old/doc/search-index.js.gz
96864   tmp/ripgrep/toolchain_new/doc/search-index.js.gz

$ du -hs tmp/{arti,cortex-m,sqlx,stm32f4,ripgrep}/toolchain_{old,new}/doc/search-index.j
s.gz
1.6M    tmp/arti/toolchain_old/doc/search-index.js.gz
1.6M    tmp/arti/toolchain_new/doc/search-index.js.gz
24K     tmp/cortex-m/toolchain_old/doc/search-index.js.gz
24K     tmp/cortex-m/toolchain_new/doc/search-index.js.gz
424K    tmp/sqlx/toolchain_old/doc/search-index.js.gz
412K    tmp/sqlx/toolchain_new/doc/search-index.js.gz
4.3M    tmp/stm32f4/toolchain_old/doc/search-index.js.gz
4.2M    tmp/stm32f4/toolchain_new/doc/search-index.js.gz
108K    tmp/ripgrep/toolchain_old/doc/search-index.js.gz
104K    tmp/ripgrep/toolchain_new/doc/search-index.js.gz
```

0 files changed, 0 insertions, 0 deletions