Abstract filter system further to allow more complex nested conditional operations

[jonahseguin]

Goal is to support more complex queries like so:

db.users.find({
$or: [ { address: { country: "CA" } }, { name: "jonah" } ]
})

which would look something like this:

let filter: Filter = User::filter()
  .any(|f| {
    f.entry(f.address().country().eq("CA".to_string()))
    .entry(f.name().eq("jonah".to_string()))
  }).build();

// OR

let filter: Filter = User::filter()
  .any(|f| vec![
    f.address().country().eq("CA".to_string()), 
    f.name().eq("jonah".to_string()),
    ])
  .build();

[jonahseguin]

@bizarre Implemented this in latest commit on jonah/macro branch; see here @ musty/examples/filter.rs

Let me know what you think of the implementation or if any changes should be made