ES6 Module Syntax Sign-Off

* revision of ES6 import final grammar
* revision of ES6 export final grammar
* introduces `Syntax.ImportNamespaceSpecifier` for `import * as foo from "foo"`
* introduces `Syntax.ImportDefaultSpecifier` for `import foo from "foo";`
* introduces `ModuleSpecifier`
* left-hand side of comma in ImportDeclaration must be identifier
* right-hand side of comma in ImportDeclaration can be NameSpaceImport or NamedImports
* removes `kind=default` and `kind=named` from `ImportDeclaration` in favor of
  `Syntax.ImportDefaultSpecifier` form
* Removes `module {}` syntax
* moves all module tests into harmonymodulestest.js