refactor(core): simplify decorators

Every decorator now is made of the following:
- a function that can be used
as a decorator or as a constructor. This function
also can be used for `instanceof` checks.
- a type for this function (callable and newable)
- a type that describes the shape of the data
  that the user needs to pass to the decorator
  as well as the instance of the metadata

The docs for decorators live at the followig places
so that IDEs can discover them correctly:
- General description of the decorator is placed on the
  `...Decorator` interface on the callable function
  definition
- Property descriptions are placed on the interface
  that describes the metadata produces by the decorator

modules/@angular/core/testing/test_compiler.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Compiler, CompilerOptions, ComponentMetadataType, DirectiveMetadataType, Injector, NgModuleMetadataType, PipeMetadataType, Type} from '@angular/core';
+import {Compiler, CompilerOptions, Component, Directive, Injector, NgModule, Pipe, Type} from '@angular/core';
 
 import {unimplemented} from './facade/errors';
 import {MetadataOverride} from './metadata_override';
@@ -19,18 +19,16 @@ import {MetadataOverride} from './metadata_override';
  */
 export class TestingCompiler extends Compiler {
   get injector(): Injector { throw unimplemented(); }
-  overrideModule(module: Type<any>, overrides: MetadataOverride<NgModuleMetadataType>): void {
+  overrideModule(module: Type<any>, overrides: MetadataOverride<NgModule>): void {
     throw unimplemented();
   }
-  overrideDirective(directive: Type<any>, overrides: MetadataOverride<DirectiveMetadataType>):
-      void {
+  overrideDirective(directive: Type<any>, overrides: MetadataOverride<Directive>): void {
     throw unimplemented();
   }
-  overrideComponent(component: Type<any>, overrides: MetadataOverride<ComponentMetadataType>):
-      void {
+  overrideComponent(component: Type<any>, overrides: MetadataOverride<Component>): void {
     throw unimplemented();
   }
-  overridePipe(directive: Type<any>, overrides: MetadataOverride<PipeMetadataType>): void {
+  overridePipe(directive: Type<any>, overrides: MetadataOverride<Pipe>): void {
     throw unimplemented();
   }
 }