* Improving variable expansion runtime
The current implementation of variable expansion is O(n x m) with n being the
number of strings in the project spec and m being the number of project variables.
This implementation is now O(n).
Also, this effectively deprecates the support for $legacy_variables in favor of the
${new_variables} making this whole patch possible.
* Adding option to disable variable expansion
* Adding performance test for spec loading
* Updating changelog
