(maint) Correct spelling in comments
(PUP-30) Make epp parser emit EPP_START, and generate Program

Parsing EPP now results in a Program model (if producing only
an EppExpression there are no source code references).
The EPP_START tokten was not emitted (and its precence was not
tested for).
(PUP-30) Add tests for epp parser and make them pass

This adds tests for parsing epp. A typo was corrected when checking
for unbalanced <% %> at the end of the epp input. This caused error
not to be raised.

(Tests should have been included in an earlier commit).
(PUP-30) Add call-by-value of closure in evaluator

This is needed for calling epp templates, but is also a useful evaluator
extension in general as it allows calling any lambda with matching of
parameter names (instead of call by args position).