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.