Make the Handlebars environment into an object

The idea is that the environment wraps up the mutable stuff in
Handlebars (like the helpers) and that you could theoretically create a
new one at any time and pass it in to Handlebars.template.

Every test makes a new environment and uses it in template compilation.

lib/handlebars/compiler/compiler.js

@@ -443,17 +443,17 @@ export function compile(input, options) {
 
   var compiled;
 
-  function compile() {
+  function compileInput() {
     var ast = parse(input);
     var environment = new Compiler().compile(ast, options);
     var templateSpec = new JavaScriptCompiler().compile(environment, options, undefined, true);
-    return template(templateSpec);
+    return template(templateSpec, options.env || Handlebars, compile);
   }
 
   // Template is only compiled on first use and cached after that point.
   return function(context, options) {
     if (!compiled) {
-      compiled = compile();
+      compiled = compileInput();
     }
     return compiled.call(this, context, options);
   };