To support strings as lists

[japeto]

We can work with lists and strings. A common lists contain a number of items and common string contain a number of characters. Both can be treated as list ADT where:

• "" is empty string.
• " " is one element string, whitespaces also treated as characters.
• "abcd" is similar to cons('a', cons('b', cons('c', cons('d', empty))))

strings and lists behave so similarly.