Auto merge of #90443 - camelid:rustdoc-subst, r=GuillaumeGomez

Merge `DocContext.{ty,lt,ct}_substs` into one map

It should be impossible to have more than one entry with a particular
key across the three maps, so they should be one map. In addition to
making it impossible for multiple entries to exist, this should improve
memory usage since now only one map is allocated on the stack and heap.

r? `@GuillaumeGomez`

src/librustdoc/lib.rs

@@ -9,6 +9,7 @@
 #![feature(control_flow_enum)]
 #![feature(box_syntax)]
 #![feature(in_band_lifetimes)]
+#![feature(let_else)]
 #![feature(nll)]
 #![feature(test)]
 #![feature(crate_visibility_modifier)]