Potential (but highly unlikely) stack overflow

[rockymeza]

If you have a super deeply nested expression, the Evaluator#resolve method is susceptible to stack overflows.

We could rewrite using a stack and a while loop to avoid this problem. (This may make it easier to build a stack trace for error messages too)

here's a failing test:

conflisp-ruby/spec/conflisp/evaluator_spec.rb

Lines 23 to 32 in eed1adc

	# The Evaluator#resolve method is susceptible to stack overflows.
	# The test below uncovers that.
	#
	# it 'can evaluate nested expressions of arbitrary depth' do
	# expression = ['add', 0, 1]
	# 3843.times do
	# expression = ['add', expression, 1]
	# end
	# expect(evaluator.resolve(expression)).to eq(3844)
	# end

Notes:

• Explanation of inorder tree traversal using a loop and a stack. https://www.geeksforgeeks.org/inorder-tree-traversal-without-recursion/
• Example of a stack and a loop in ruby parsing an AST https://gitlab.com/inko-lang/inko/blob/master/compiler/lib/inkoc/parser.rb#L184