Implement defun

[tshemsedinov]

Function declaration: (defun square (x) (* x x))

[zpwebbear]

@tshemsedinov I just want to point out that it might not be as easy as it seems because of the following functions features:

• documentation string (defun square (x) "Multiplying a number by itself." (* x x))
• optional parameters (defun foo (a b &optional c d) (list a b c d))
• rest parameters (defun format (stream string &rest values) ...)
• keyword parameters (defun foo (&key a b c) (list a b c))
• mixing different parameter types (defun foo (x &optional y &key z) (list x y z))

I'm also in doubts about to include lambda functions in this issue or not to include.