rustdoc: add optional woff2 versions of FiraSans.

For browsers that support woff2 (most modern ones:
https://caniuse.com/woff2), this offers a reduction in download size
for these two fonts from 362k to 257k (32% reduction). It decreases the
total page size for `struct.String.html` (counting all subresources) by
about 2.5%.

If this is interesting, I'm happy to apply the same treatment to the
other fonts, but these two are the biggest.

rustdoc: add optional woff2 versions of FiraSans.

For browsers that support woff2 (most modern ones:
https://caniuse.com/woff2), this offers a reduction in download size
for these two fonts from 362k to 257k (32% reduction). It decreases the
total page size for `struct.String.html` (counting all subresources) by
about 2.5%.

If this is interesting, I'm happy to apply the same treatment to the
other fonts, but these two are the biggest.

…laumeGomez

Rollup of 8 pull requests

Successful merges:

 - rust-lang#81940 (Stabilize str_split_once)
 - rust-lang#82165 (Reword labels on E0308 involving async fn return type)
 - rust-lang#82456 (Replaced some unwrap_or and map_or with lazy variants)
 - rust-lang#82491 (Consider inexpensive inlining criteria first)
 - rust-lang#82506 (Properly account for non-shorthand pattern field in unused variable lint)
 - rust-lang#82535 (Set codegen thread names)
 - rust-lang#82545 (rustdoc: add optional woff2 versions of FiraSans.)
 - rust-lang#82549 (Revert "Update normalize.css to 8.0.1")

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

rustdoc: add optional woff2 versions of Source Serif and Source Code

This provides woff2 versions of Source Serif and Source Code similar to how rust-lang#82545 provides woff2 versions of Fira Sans. The total byte count for the six files (three for each font family) is reduced by 25% from 476 KiB to 358 KiB.

rustdoc: add optional woff2 versions of Source Serif and Source Code

This provides woff2 versions of Source Serif and Source Code similar to how rust-lang#82545 provides woff2 versions of Fira Sans. The total byte count for the six files (three for each font family) is reduced by 25% from 476 KiB to 358 KiB.