Improving variable expansion runtime (#704)

* 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