[Yaml] Optimise Inline::evaluateScalar() for parsing strings.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

The Drupal 8 installer does a lot of YAML parsing. With the patch attached it is significantly quicker.
![image](https://f.cloud.github.com/assets/769634/2237906/630c3308-9bcf-11e3-8038-35cbbeedd7e6.png)

https://drupal.org/node/1851234