Inline `Span` methods.
Inline `Span` methods.

Because they are simple and hot.

This change speeds up some incremental runs of a few rustc-perf
benchmarks, the best by 3%.

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