Provide option to set the reverse of an association

It is an extremely common pattern to have hierarchical associations of this form:
```crystal
belongs_to page : Page?
has_many pages : Page
```
The main query in my application has to preload reverse associations that it should be able to simply fill in, thus extra database queries.

I would like for associations to be extended with an optional **reverse:** argument, of this form:
```crystal
belongs_to page : Page?, reverse: pages
has_many pages : Page, reverse: page
```
This would allow us to do several nice things:
* Check that the reverse association exists.
* Fill in the reverse association when creating a query result (see #389)
* Avoid an extra query to fill in the reverse association.

No files or tests are named. Start by tracing how belongs_to and has_many declarations are handled and how query results preload associations; review #389 for the related result-loading context. Done means reverse associations can be declared, checked, populated from query results, and avoid the extra query described here.

Written by the indexing model from the issue text.