fix: printing Var(27) and beyond overflowed ASCII

[AgentElement]

For example, printing Var(27) spits out }, because the character conversion function was basically char(96 + index), thereby overflowing the alphanumeric ASCII section. I've fixed this by encoding the lambda variables in a base 26 string.

Any programs that print out lambda expressions with Var(26) or lower should remain unchanged in behavior.

[ljedrz]

I had wondered if someone would ever need that many variables to display, but I'm all for making this possible 😃; I left some suggestions and a question.

[AgentElement]

Everything should be resolved now, except for the inverted comparison - I think it makes sense to view depth as an upper bound on i, and move it to the right. I was already changing the original line, so I figured it didn't matter too much.

[ljedrz]

LGTM 👌