Right now, our lowering code uses string-paths to call e.g. Iterator::next when lowering for loops:

This triggers a name resolution call

which will then give us a resolved path, but that's

1. potentially expensive, since it happens every time a specific path is generated (which happens a lot for ? and for expansions/lowerings)
2. seems uncool since it's stringly typed

I think it would be neat if we could instead create the lang_items table early in the process and just grab the appropriate item out of that table.

cc @Centril @eddyb @petrochenkov

As a first experiment to check out the impact we could create a cache for path resolutions so we only run the path resolution code once and just overwrite all HirIds in the generated path with new HirIds whenever we fetch a cached resolution.