Rejig some top-level rustc_hir_pretty functions.

There are several that are unused and can be removed.

And there are some calls to `to_string`, which can be expressed more
nicely as a `foo_to_string` call, and then `to_string` need not be
`pub`. (This requires adding `pat_to_string`).

clippy_lints/src/mut_reference.rs

@@ -49,7 +49,7 @@ impl<'tcx> LateLintPass<'tcx> for UnnecessaryMutPassed {
                         cx,
                         arguments.iter().collect(),
                         cx.typeck_results().expr_ty(fn_expr),
-                        &rustc_hir_pretty::to_string(rustc_hir_pretty::NO_ANN, |s| s.print_qpath(path, false)),
+                        &rustc_hir_pretty::qpath_to_string(path),
                         "function",
                     );
                 }