Rollup merge of #50564 - nnethercote:inline-Span-methods, r=petrochenkov
Rollup merge of #50564 - nnethercote:inline-Span-methods, r=petrochenkov

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%.

Here are the ones with a speedup of at least 1%:
```
coercions
        avg: -1.1%      min: -3.4%      max: -0.2%
html5ever-opt
        avg: -0.8%      min: -1.7%      max: -0.2%
clap-rs-check
        avg: -0.3%      min: -1.4%      max: 0.7%
html5ever
        avg: -0.7%      min: -1.2%      max: -0.4%
html5ever-check
        avg: -0.9%      min: -1.1%      max: -0.8%
clap-rs
        avg: -0.4%      min: -1.1%      max: -0.1%
crates.io-check
        avg: -0.8%      min: -1.0%      max: -0.6%
serde-opt
        avg: -0.6%      min: -1.0%      max: -0.3%
```

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