Define `DefCollector` in HIR lowering section

[rylev]

In the section on AST to HIR lowering, the DefCollector is mentioned without explanation. We should talk more about what it is.

[jyn514]

There are autogenerated docs at https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/def_collector/struct.DefCollector.html.