#7081: Further syntax::visit refactors

r? anyone.

Part of #7081.

More refactorings of the syntax::visit::Visitor implementations, folding so-called "environments" into the visitor impl when the latter was previously a trivial unit struct.

As usual, this refactoring only applies when the environments are not actually carrying state that is meant to be pushed and popped as we traverse the expression.  (For an example where the environment *isn't* just passed through, see the `visit_fn` in `liveness.rs`.)

Got rid of a bit of @-allocation in borrowck.

Both cases should be pure-refactorings.