ast+topdown: exit-early for (constant) complete virtual docs and functions (#3898)

If the following conditions hold for a set of rules returned by the indexer,
it will set EarlyExit: true, and change how the complete virtual doc or
function is evaluated:

- all rule head values are ground
- all rule head values match

This implies that some cases where early exit would be possible will not be
covered:

    p = x {
      x := true
      input.foo == "bar"
    }

    p = x {
      x := true
      input.baz == "quz"
    }

To indicate that "early exit" is possible, the indexer result message is
amended. Also, the "Exit" trace event will have a message of "early" when
"early exit" actually happens in eval:

    $ echo '{"x":"x", "y":"y"}' | opa eval -I -fpretty --explain=full -d r.rego data.r.r
    query:1       Enter data.r.r = _
    query:1       | Eval data.r.r = _
    query:1       | Index data.r.r (matched 2 rules, early exit)
    r.rego:11     | Enter data.r.r
    r.rego:12     | | Eval input.y = "y"
    r.rego:11     | | Exit data.r.r
    query:1       | Exit data.r.r = _
    query:1       Redo data.r.r = _
    query:1       | Redo data.r.r = _
    r.rego:11     | Redo data.r.r
    r.rego:12     | | Redo input.y = "y"
    r.rego:11     | Exit data.r.r early

With `r.rego` as

    package r
    r {
      input.x = "x"
    }
    r = 2 {
      input.z = "z"
    }
    r {
      input.y = "y"
    }

This is done in in a way such that early-exit will abort array/set/object
iterations on data:

    r {
      data.i[_] = "one"
      data.j[_] = "four"
    }

    f(x, y) {
      data.i[_] = x
      data.j[_] = y
    }

Complete rules (r) and functions (f) that iterate over sets, arrays, and
objects from either data (evalTree) or a term that's returned by some
other rule etc (evalTerm).

The CLI and golang packages expose ways to disable 'early-exit':

This is in line with how indexing can be disabled. It's supposed to be
used as a debugging measure, so it's only exposed as a CLI flag to
`opa eval`.

Co-authored-by: Torin Sandall <torinsandall@gmail.com>
Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>