feat(dispose): dispose child injectors

[nicojs]

This changes the way dispose works. Instead of an Injector disposing it's own provided dependencies and then call dispose on the parent, it will dispose its child Injector's (that were created using provideXXX) and then it's own dependencies. The parent Injector will no longer be disposed of.

This allows you to dispose from the rootInjector down:

class Foo {
  dispose() {
    console.log('Foo disposed');
  }
}
const rootInjector = createInjector();
const fooProvider = rootInjector.provideClass('foo', Foo);
fooProvider.resolve('foo');
await rootInjector.dispose(); // => "Foo disposed"
rootInjector.provideClass('bar', Bar); // Error: Injector already disposed

Because now an Injector has to keep track of its child injectors, the rootInjector would no longer be stateless. That's why it is removed altogether. Developers are expected to create their own (root) injector using createInjector().

BREAKING CHANGE: rootInjector is removed in favor of createInjector.

This:

import { rootInjector } from 'typed-inject';

Becomes:

import { createInjector } from 'typed-inject';
const rootInjector = createInjector();

Injector's created from createInjector are no longer stateless. They
keep track of their child injectors.

BREAKING CHANGE: dispose no longer disposes parent injector, disposes
the child injectors instead. See readme for more details.

Fixes #24