Lazier core filters

[01mf02]

This PR delays the execution of many core filters.
For example, before this change, the filter 1, debug printed:

[DEBUG] null
1
null

After this change, this prints:

1
[DEBUG] null
null

This is more consistent with jq, which yields:

1
["DEBUG:",null]
null

In some cases, this makes filters terminate which did not terminate before. For example,

[0, 1] | first(1, sort_by(repeat(0)))

yielded no single output before, because sort_by was executed before 1 was yielded. With this PR, jaq yields 1 (just like jq).

Previously, also the last/1 filter was executed strictly. With this PR, it is implemented by definition in a naturally lazy way.
This makes the bf-fib benchmark a bit slower (runtime decreases from 414.8 ms to 485.3 ms), but it remains still a lot faster than in jq or gojq. This also makes the evaluation fairer, because jq also implements last/1 by definition.