Auto merge of #147002 - notriddle:stringdex3, r=GuillaumeGomez

rustdoc-search: stringdex update with more packing

Before:

    18M  build/x86_64-unknown-linux-gnu/doc/search.index/
    57M  build/x86_64-unknown-linux-gnu/compiler-doc/search.index/

After:

    16M  build/x86_64-unknown-linux-gnu/doc/search.index/
    49M  build/x86_64-unknown-linux-gnu/compiler-doc/search.index/

CC rust-lang/rust#146063

Author: bors