WIXFEAT:4299 - Improve security for hidden burn variables

Implements [WIXFEAT:4299](http://wixtoolset.org/issues/4299/).

Based on the code from the [Codeplex pull request](https://wix.codeplex.com/SourceControl/network/forks/seanhall/working/contribution/6219).  The first commit here contains the Codeplex code.  The next commit removed the IBootstrapperEngineSecure file.  The next commit removed the duplicated code in strutil.  We might need to revisit if this can be taken in 3.x.