Add test for define: identity function

'(define (f x) x)' tested on strings, numbers and booleans.
Treat source between quotation marks as single unit

utility.find_end now has a third mode, when the first character is
either a single or double quotation mark (and in this case the return is
the closing quotation mark).
Add string test for identity function
Add replace_all_plural function

Takes (str, list, to) and iterates through list, calling
replace_all(str, list_element, to) for each list_element in list.
Add additional string test for identity function