Micro-optimize rustdoc search index parsing

[yotamofek]

This should pre-allocate the correct size for the slot vec. Not sure how much of a difference it'll make, but let's see.

[rustbot]

r? @GuillaumeGomez

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[yotamofek]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[rust-bors]

☀️ Try build successful (CI)
Build commit: 68a9c0e (68a9c0e6ecf9481452ec6e82f8b0675cd643fd63, parent: 8e0b68e63cd2b7b6d18474fe6f49df6fb1570c25)

[rust-timer]

Queued 68a9c0e with parent 8e0b68e, future comparison URL.
There are currently 2 preceding artifacts in the queue.
It will probably take at least ~3.9 hours until the benchmark run finishes.