rustdoc: use restricted Damerau-Levenshtein distance for search

Based on rust-lang#108200, for the same
rationale.

> This replaces the existing Levenshtein algorithm with the
> Damerau-Levenshtein algorithm. This means that "ab" to "ba" is one change
> (a transposition) instead of two (a deletion and insertion). More
> specifically, this is a restricted implementation, in that "ca" to "abc"
> cannot be performed as "ca" → "ac" → "abc", as there is an insertion in the
> middle of a transposition. I believe that errors like that are sufficiently
> rare that it's not worth taking into account.

Before this change, searching `prinltn!` listed `print!` first, followed
by `println!`. With this change, `println!` matches more closely.

Author: notriddle