ENH: improvements to stringOps::expand operations

- add toScalar evaluation, embedded as "${{EXPR}}".

  For example,

    "repeat ${{5 * 7}} times or ${{ pow(3, 10) }}"

- use direct string concatenation if primitive entry is only a string
  type. This prevents spurious quotes from appearing in the expansion.

     radius  "(2+4)";
     angle   "3*15";
     #eval   "$radius*sin(degToRad($angle))";

     We want to have
         '(2+4)*sin(degToRad(3*15))'
     and not
         '"(2+4)"*sin(degToRad("3*15"))'

ENH: code refactoring

- refactored expansion code with low-level service routines now
  belonging to file-scope. All expansion routines use a common
  multi-parameter backend to handle with/without dictionary etc.
  This removes a large amount of code duplication.