Added a perf test to check that the caching works.

* Uses `matchAll()`, which runs repeated operations on the same string.
* Changing the string size should exhibit a near linear behavior.
* (With high numbers we measure Node's GC and had to recreate the cache).

tests/manual/matchall-bench.js

@@ -0,0 +1,16 @@
+'use strict';
+
+const RE2 = require('../../re2');
+
+const N = 1_000_000;
+
+const s = 'a'.repeat(N),
+  re = new RE2('a', 'g'),
+  matches = s.matchAll(re);
+
+let n = 0;
+for (const _ of matches) ++n;
+
+if (n !== s.length) console.log('Wrong result.');
+
+console.log('Done.');