Remove NLL feature

And do some refactoring in comments.rs

Closes #3107

Author: Nick Cameron

src/chains.rs

@@ -447,7 +447,7 @@ trait ChainFormatter {
     // Parent is the first item in the chain, e.g., `foo` in `foo.bar.baz()`.
     // Root is the parent plus any other chain items placed on the first line to
     // avoid an orphan. E.g.,
-    // ```
+    // ```ignore
     // foo.bar
     //     .baz()
     // ```
@@ -509,7 +509,7 @@ impl<'a> ChainFormatterShared<'a> {
     // know whether 'overflowing' the last child make a better formatting:
     //
     // A chain with overflowing the last child:
-    // ```
+    // ```ignore
     // parent.child1.child2.last_child(
     //     a,
     //     b,
@@ -518,7 +518,7 @@ impl<'a> ChainFormatterShared<'a> {
     // ```
     //
     // A chain without overflowing the last child (in vertical layout):
-    // ```
+    // ```ignore
     // parent
     //     .child1
     //     .child2
@@ -527,7 +527,7 @@ impl<'a> ChainFormatterShared<'a> {
     //
     // In particular, overflowing is effective when the last child is a method with a multi-lined
     // block-like argument (e.g. closure):
-    // ```
+    // ```ignore
     // parent.child1.child2.last_child(|a, b, c| {
     //     let x = foo(a, b, c);
     //     let y = bar(a, b, c);