Move $exists to the new model

Fixes two incompatibilities:
- {$exists: false} did the wrong logic when there were multiple branches
   (we (poorly) special-cased $not and $nin but not this negative case)
- No longer require the argument to $exists to be a boolean;
  {$exists: 0} and {$exists: 1} should work, eg

packages/minimongo/minimongo_tests.js

@@ -433,6 +433,15 @@ Tinytest.add("minimongo - selector_compiler", function (test) {
   nomatch({a: {$exists: false}}, {a: [1]});
   match({a: {$exists: false}}, {b: [1]});
 
+  match({a: {$exists: 1}}, {a: 5});
+  match({a: {$exists: 0}}, {b: 5});
+
+  nomatch({'a.x':{$exists: false}}, {a: [{}, {x: 5}]});
+  match({'a.x':{$exists: true}}, {a: [{}, {x: 5}]});
+  match({'a.x':{$exists: true}}, {a: [{}, {x: 5}]});
+  match({'a.x':{$exists: true}}, {a: {x: []}});
+  match({'a.x':{$exists: true}}, {a: {x: null}});
+
   // $mod
   match({a: {$mod: [10, 1]}}, {a: 11});
   nomatch({a: {$mod: [10, 1]}}, {a: 12});

packages/minimongo/selector.js

@@ -258,6 +258,12 @@ var VALUE_OPERATORS = {
   $nin: function (operand) {
     return invertBranchedSelector(convertElementSelectorToBranchedSelector(
       ELEMENT_OPERATORS.$in(operand)));
+  },
+  $exists: function (operand) {
+    var exists = convertElementSelectorToBranchedSelector(function (value) {
+      return value !== undefined;
+    });
+    return operand ? exists : invertBranchedSelector(exists);
   }
 };
 
@@ -1038,7 +1044,6 @@ var andCompiledDocumentSelectors = function (selectors) {
 
 
 // Remaining to update:
-// - $exists
 // - $type
 // - $regex/$option
 // - $all