Replaced two-level rule cache with a one-level one.

This leads to simpler code and also speeds up the benchmark suite execution by
5,89 % on V8.

Detailed results (benchmark suite totals):

---------------------------------
 Test #     Before       After
---------------------------------
      1   24,70 kB/s   26,14 kB/s
      2   24,49 kB/s   26,05 kB/s
      3   24,67 kB/s   25,99 kB/s
      4   24,65 kB/s   26,13 kB/s
      5   24,71 kB/s   26,14 kB/s
---------------------------------
Average   24,64 kB/s   26.10 kB/s
---------------------------------

Mozilla/5.0 (X11; U; Linux i686; en-US) AppleWebKit/533.2 (KHTML, like Gecko) Chrome/5.0.342.7 Safari/533.2

lib/compiler.js

@@ -394,8 +394,8 @@ PEG.Grammar.Rule.prototype.compile = function() {
 
   return PEG.Compiler.formatCode(
     "result._parse_${name} = function(context) {",
-    "  this._cache[${name|string}] = this._cache[${name|string}] || [];",
-    "  var cachedResult = this._cache[${name|string}][this._pos];",
+    "  var cacheKey = ${name|string} + '@' + this._pos;",
+    "  var cachedResult = this._cache[cacheKey];",
     "  if (cachedResult !== undefined) {",
     "    this._pos = cachedResult.nextPos;",
     "    return cachedResult.result;",
@@ -408,7 +408,7 @@ PEG.Grammar.Rule.prototype.compile = function() {
     "  ${restoreReportMatchFailuresCode}",
     "  ${reportMatchFailureCode}",
     "  ",
-    "  this._cache[${name|string}][pos] = {",
+    "  this._cache[cacheKey] = {",
     "    nextPos: this._pos,",
     "    result:  ${resultVar}",
     "  };",