src/compiler.js: Fix variable redefinition

Fixes the following JSHint error:

  ./src/compiler.js: line 23, col 16, 'i' is already defined.

src/compiler.js

@@ -16,11 +16,13 @@ PEG.compiler = {
       "stackDepths"
     ];
 
-    for (var i = 0; i < CHECK_NAMES.length; i++) {
+    var i;
+
+    for (i = 0; i < CHECK_NAMES.length; i++) {
       this.checks[CHECK_NAMES[i]](ast);
     }
 
-    for (var i = 0; i < PASS_NAMES.length; i++) {
+    for (i = 0; i < PASS_NAMES.length; i++) {
       ast = this.passes[PASS_NAMES[i]](ast);
     }