Return inserted/replaced paths (#5710)

* Return inserted/replaced paths

This gives `Path`’s replacement and insertion methods a consistent
return value: the inserted/replaced paths.

Before, they could return `undefined`, a `node`, or a the current path
inside an array. It was kinda pointless.  But now they always return an
array of paths, which is useful for solving
https://github.com/babel/babel/pull/4935#discussion_r96151368.

* Return inserted nodes and not BlockStatement

Addded test for bug #4363

* Cleanups

- `#replaceWith` will now return the current path if it's the same node
- `#insertAfter` and `#insertBefore` use public Path APIs now
- Makes container insertion faster (single splice call)
- Use public APIs in container insertion
- Replacing a statement with an expression returns the expression's path
- Replacing an expression with multiple statements returns the inserted
  closure's body's paths.

packages/babel-plugin-transform-es2015-classes/src/index.js

@@ -44,7 +44,10 @@ export default function({ types: t }) {
         if (node[VISITED]) return;
 
         const inferred = nameFunction(path);
-        if (inferred && inferred !== node) return path.replaceWith(inferred);
+        if (inferred && inferred !== node) {
+          path.replaceWith(inferred);
+          return;
+        }
 
         node[VISITED] = true;