check_loans.rs Visitor refactoring (#7081): unify CheckLoan{Ctxt,Visitor}.
gather_loans/mod.rs Visitor refactoring (#7081): unify GatherLoan{Ctxt,Visitor}.
middle/entry.rs Visitor refactoring (#7081): unify Entry{Context,Visitor}.
auto merge of #9094 : pnkfelix/rust/fsk-visitor-ports, r=huonw

r? anyone

Remove some trivial Visitor structs, using their non-trivial Contexts as the Visitor implementation instead.

Removed a little bit of `@boxing` as well.

Part of ongoing work on #7081.