Auto merge of #96260 - Kobzol:rustdoc-idmap, r=petrochenkov

rustdoc: Optimize IdMap

Slightly optimizes `IdMap`, which is hot in `markdown_links` (context [here](https://github.com/rust-lang/rust/pull/96135#issuecomment-1103539052)). There are more improvements that can be made near this place, but this seemed like an easy win locally (although I tried it on top of https://github.com/rust-lang/rust/pull/94857, so let's see what happens without that PR).

r? `@petrochenkov`

Cargo.lock

@@ -2531,9 +2531,9 @@ dependencies = [
 
 [[package]]
 name = "once_cell"
-version = "1.7.2"
+version = "1.10.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "af8b08b04175473088b46763e51ee54da5f9a164bc162f615b91bc179dbf15a3"
+checksum = "87f3e037eac156d1775da914196f0f37741a274155e34a0b7e427c35d2a2ecb9"
 
 [[package]]
 name = "opaque-debug"
@@ -4536,6 +4536,7 @@ dependencies = [
  "expect-test",
  "itertools",
  "minifier",
+ "once_cell",
  "pulldown-cmark",
  "rayon",
  "regex",