Reduce calls to :entity method with instance_vars

[ashmaroli]

Similar to what was already proposed in #18 but with the following differences:

• employ instance_variables instead of "temporary" local variables.
• make the substitution on all occurrences instead of just subs in a single method.
• have a cleaner diff for easier review
  • less error-prone
  • @ can also be seen as a informal shorthand alias for entity()