Add functions and function calls to your MSDscript implementation, including the parser, interpreter, and printer.

• You can remove the has_variable method from your expression class, since it's not useful.
• For print, always put a _fun expression in parentheses, include exactly one space around the outside of the parentheses for the argument, and include no space inside the parentheses. Never print a pair of parentheses around a function call (because it turns out that's not necessary), and include no space on the outside or inside edges of the parentheses around the argument in a function call.
• For pretty_print, if you have it, _fun expressions need parentheses in the same contexts where _let would need parentheses. Add a newline after the argument parentheses for _fun and indent the body two spaces more than the column where _fun starts. A function call never needs extra parentheses. As an example, the implementation of factorial below is correctly pretty-printed.

Here's a program that you can try out on your interpreter to compute 10!, which is 3628800:

_let factrl = _fun (factrl)
                _fun (x)
                  _if x == 1
                  _then 1
                  _else x * factrl(factrl)(x + -1)
_in  factrl(factrl)(10)