Improve Lisp binding

[vinc]

• Move eval::eval_args calls out of env::inner_env in interpreter
• Remove env::function_env and env::macro_env in interpreter
• Rename env::inner_env to env::bind in interpreter
• Allow optional arguments
• Add eval::apply in interpreter
• Add back apply special form without double eval bug
• Add fold special form
• Update doc

[vinc]

If we dissociate function arguments evaluation from parameters binding we don't need a special case for macros, so we can merge function_env and macro_env into bind and simplify the code.

[vinc]

We couldn't have optional args before:

> ((fun (a b @opts) opts) 1 2 3 4)
(3 4)

> ((fun (a b @opts) opts) 1 2)
Error: Expected at least 3 arguments, got 2

This is now fixed:

> ((fun (a b @opts) opts) 1 2 3 4)
(3 4)

> ((fun (a b @opts) opts) 1 2)
()

[vinc]

This last commit change the evaluation order but I think the new order is fine:

Before:

> (+ 1 2 3)
6

> (a 1 2 3)
Error: Could not find symbol 'a'

> (1 2 3)
Error: Expected first argument to be a function

> (1 2 a)
Error: Expected first argument to be a function

After:

> (+ 1 2 3)
6

> (a 1 2 3)
Error: Could not find symbol 'a'

> (1 2 3)
Error: Expected first argument to be a function

> (1 2 a)
Error: Could not find symbol 'a'

[vinc]

Now that bind is no longer evaluating the function arguments it is possible to reintroduce the apply special form without the double eval issue from before.

This works now:

> (apply (fun (x) x) (list '(1 2)))
(1 2)

[vinc]

With the apply helper it was straightforward to add a (left) fold special form:

> (fold - 0 '(1 2 3))
-6

[vinc]

With fold we can now fix reduce:

> (reduce - '(1 2 3))
-4

> (reduce - '(1 2))
-1

> (reduce - '(1))
1

> (reduce - '())
()