rustdoc: Use smaller sequential numbers instead of `NodeId`s for parents.

`allPaths` is now a flat array in effect. This decreases the size of
the search index by about 4--5% (gzipped or not).
rustdoc: Represent item types as a small number in the search index.

Has negligible improvements with gzip, but saves about 7% without it.
This also has an effect of changing the tie-breaking order of item types.
rustdoc: Use an array instead of an object for the search index.

`buildIndex` JS function recovers them into the original object form.
This greatly reduces the size of the uncompressed search index (27%),
while this effect is less visible after gzipped (~5%).
rustdoc: Get rid of `allPaths` global variable by merging it into `searchIndex`.
rustdoc: Omit repeated paths in the search index.

Since the items roughly follow the lexical order, there are
many consecutive items with the same path value which can be
easily compressed.

For the library and compiler docs, this commit decreases
the index size by 26% and 6% before and after gzip, respectively.
auto merge of #13431 : lifthrasiir/rust/rustdoc-smaller-index, r=alexcrichton

This is a series of inter-related commits which depend on #13402 (Prune the paths that do not appear in the index). Please consider this as an early review request; I'll rebase this when the parent PR get merged and rebase is required.

----

This PR aims at reducing the search index without removing the actual information. In my measurement with both library and compiler docs, the search index is 52% smaller before gzipped, and 16% smaller after gzipped:

```
 1719473 search-index-old.js
 1503299 search-index.js (after #13402, 13% gain)
  724955 search-index-new.js (after this PR, 52% gain w.r.t. #13402)

  262711 search-index-old.js.gz
  214205 search-index.js.gz (after #13402, 18.5% gain)
  179396 search-index-new.js.gz (after this PR, 16% gain w.r.t. #13402)
```

Both the uncompressed and compressed size of the search index have been accounted. While the former would be less relevant when #12597 (Web site should be transferring data compressed) is resolved, the uncompressed index will be around for a while anyway and directly affects the UX of docs. Moreover, LZ77 (and gzip) can only remove *some* repeated strings (since its search window is limited in size), so optimizing for the uncompressed size often has a positive effect on the compressed size as well.

Each commit represents the following incremental improvements, in the order:

1. Parent paths were referred by its AST `NodeId`, which tends to be large. We don't need the actual node ID, so we remap them to the smaller sequential numbers. This also means that the list of paths can be a flat array instead of an object.
2. We remap each item type to small predefined numbers. This is strictly intended to reduce the uncompressed size of the search index.
3. We use arrays instead of objects and reconstruct the original objects in the JavaScript code. Since this removes a lot of boilerplates, this affects both the uncompressed and compressed size.
4. (I've found that a centralized `searchIndex` is easier to handle in JS, so I shot one global variable down.)
5. Finally, the repeated paths in the consecutive items are omitted (replaced by an empty string). This also greatly affects both the uncompressed and compressed size.

There had been several unsuccessful attempts to reduce the search index. Especially, I explicitly avoided complex optimizations like encoding paths in a compressed form, and only applied the optimizations when it had a substantial gain compared to the changes. Also, while I've tried to be careful, the lack of proper (non-smoke) tests makes me a bit worry; any advice on testing the search indices would be appreciated.