Rollup merge of #53644 - llogiq:smallvec-for-small-c-str, r=estebank
Rollup merge of #53644 - llogiq:smallvec-for-small-c-str, r=estebank

Use SmallVec for SmallCStr

This reuses the awesome optimizations from Servo's `SmallVec` to speed up `SmallCStr`.

GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust