feat(core): refactor merging to allow querying by custom type

[B4nan]

This changes internal mechanism of em.merge(). Originally it was using
EntityAssigner inside em.merge() together with UoW.merge(), which
was called twice. Because of this, the custom types were processed multiple
times, and the value passed to conversion method was often already converted.

Now we use only the Hydrator for this. As a sideproduct of those changes,
we can now query by custom types.

class UuidBinaryType extends Type<string, Buffer> {

  convertToDatabaseValue(value: string): Buffer {
    return Buffer.from(parse(value));
  }

  convertToJSValue(value: Buffer): string {
    return stringify(value);
  }

  getColumnType(): string {
    return 'binary(16)';
  }

}

@Entity()
class A {

  @PrimaryKey({ type: UuidBinaryType })
  id: string = v4();

}

const a = await em.find(A, { id: '...' });

Closes #739