Generate objects lazily & remove itertools dependency

[01mf02]

This PR makes evaluation of object keys/values lazy.

Previously, the following filter would never yield a single result:

'{("a", "b"): (0 | recurse(.+1)), ("c", "d"): 2}'

Now, it generates an infinite stream of objects, just like jq.

How's performance? Let's test it with the following filter:

limit(1000000; {("a", "b"): (0 | recurse(.+1)), ("c", "d"): 2}) | empty

jq 1.7 takes 1.1 seconds, whereas jaq takes only 0.5 seconds! :)

As a bonus, this also eliminates the dependency of jaq-interpret on itertools.