Rollup merge of #88711 - Mark-Simulacrum:fix-dfs-bug, r=jackh726

Rollup merge of #88711 - Mark-Simulacrum:fix-dfs-bug, r=jackh726

Rework DepthFirstSearch API

This expands the API to be more flexible, allowing for more visitation patterns
on graphs. This will be useful to avoid extra datasets (and allocations) in
cases where the expanded DFS API is sufficient.

This also fixes a bug with the previous DFS constructor, which left the start
node not marked as visited (even though it was immediately returned).

Commit written by ```@nikomatsakis``` originally, cherry picked from several commits in work on never type stabilization, but stands alone.