isKeyword optimization for non-keyword fast path.

- Add non-keyword fast path check if first char is '@'.
- Avoids full `switch` for data that can't be a keyword.
- Array access will just be undefined for empty string.
- Performance change depends on data shape, format, and processing
  algorithm workload. Some synthetic tests with ~80% non-keywords could
  see 1-3%+ overall increase.

Author: davidlehn

CHANGELOG.md

@@ -5,6 +5,9 @@
 - JSON literal value handling issues (`null` and `[]`).
 - Fix resolving context `null` values.
 
+### Changed
+- `isKeyword()` optimization for non-keyword fast path.
+
 ## 2.0.1 - 2019-12-10
 
 ### Fixed

lib/context.js

@@ -1156,7 +1156,7 @@ api.processingMode = (activeCtx, version) => {
  * @return true if the value is a keyword, false if not.
  */
 api.isKeyword = v => {
-  if(!_isString(v)) {
+  if(!_isString(v) || v[0] !== '@') {
     return false;
   }
   switch(v) {