feat(core): type safe references

[B4nan]

EM now returns Loaded<T, P> instead of the entity (T). This type automatically
adds synchronous method get() that returns the entity (for references) or array
of entities (for collections).

There is also $ property that contains the same as return value of this get()
method.

const book1 = await orm.em.findOneOrFail(Book, bible, { populate: ['publisher', 'tags'] });
expect(book1.publisher!.$.name).toBe('Publisher 123');
expect(book1.tags.$[0].name).toBe('t1');
expect(book1.tags.$[1].name).toBe('t2');
expect(book1.tags.$[2].name).toBe('t3');
orm.em.clear();

const books = await orm.em.find(Book, { id: bible.id }, { populate:
  { publisher: { books: { publisher: true } } },
});
expect(books[0].publisher!.$.books.$[0].publisher!.$.name).toBe('Publisher 123');

const book5 = await orm.em.findOneOrFail(Book, bible, { populate:
  { publisher: true, tags: true, perex: true },
});
expect(book5.publisher!.$.name).toBe('Publisher 123');
expect(book5.tags.$[0].name).toBe('t1');

Closes #214

BREAKING CHANGE:
Reference.get() is now available only with correct Loaded type hint and is used
as a sync getter for the entity, just like unwrap(). You can use Reference.load(prop)
for the original get() method functionality.

em.find() and similar methods now have two type arguments, due to TypeScript not supporting
partial type inference, it is no longer possible to specify the T explicitly (without also
explicitly specifying the load hint).

[lgtm-com]

This pull request introduces 1 alert when merging da33d0d into de5e66d - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[lgtm-com]

This pull request introduces 1 alert when merging 95ccebe into de5e66d - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[lgtm-com]

This pull request introduces 1 alert when merging 4b57cf4 into de5e66d - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class