Review request for JDK-8006408

[Hannes Wallnoefer]

Please review patch for JDK-8006408:

http://cr.openjdk.java.net/~hannesw/8006408/

This adds int/double/missing args specializations for the charAt, 
charCodeAt, indexOf, slice, substring string methods, and an optimized 
version of String.prototype.split() when called with a string rather 
than a regexp as separator argument.

It also contains a string micro-benchmark script.

Hannes