Clj-frontend: Allow $[XXX] substitution in inline vars

This pull request adds substitution to variables declared inline in clj-frontend.  Currently, variables declared as a separate step using the set-var function have their keys and values substituted according to $[XXX] placeholders.  This commit brings substitution to variables declared inline.

Inline variables with $[xxx] placeholders will now be substituted based
on the workflow level variables currently defined.  Modifies
step-vars-test and method-vars-test to test the substitution.

Other small changes:
Move varargs->map to the clj-frontend-utils ns
Remove the superfluous var-sub->str function.  
Replace occurrences of var-sub->str with var-sub.  var-sub now returns a string.