Auto merge of #149694 - lolbinarycat:alloc-extend-slice-of-str-spec, r=Mark-Simulacrum

alloc: specialize String::extend for slices of str

Here's a small optimization via specialization that can potentially eliminate a fair few reallocations when building strings using specific patterns, unsure if this justifies the use of unsafe, but I had the code implemented from rust-lang/rust#148604, so I thought it was worth submitting to see.

Author: bors