Conditionals and Logical Operators

[granawkins]

As part of the ongoing redesign, I'd like to optimize if and if else operations. This is also discussed in #27 and there is some good guidance there.

A logical tree should be able to express something like "If volume is above 100, price squared over 200, else short / 300." In a poorly-rendered tree structure that looks like:

               IF
     >         /          /
 vol  100   **  200  short  300
        price 2

As I see it, there ought to be three classes of nodes: operators, terminals and booleans(new). In addition to arity, different types of nodes now specify that their children be a certain class.

Class	Type	Symbol	Arity	Child Class
operator	arithmetic	+, -, *, /, **	2	op/tm
operator	logic	IF, IFELSE	2, 3	bool, op/tm
boolean	comparison	==, !=, >, <, >=, <=	2	op/tm
boolean	logic	AND, OR, NOT	2, 1	bool
terminal	inputs	a, b, c	0	-
terminal	constants	1, 2, 3	0	-

[granawkins]

This was implemented in #85