Add reverse polish notation printer for AST

This is meant to complete chapter 5 challenge 3.

pj1-go/ast-rpn.go

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"fmt"
+)
+
+func rpn(expr Expr) string {
+	switch e := expr.(type) {
+	case Binary:
+		return rpn(e.left) + " " + rpn(e.right) + " " + e.operator.Lexeme
+	case Grouping:
+		return rpn(e.expression) // + " group"    arguable if this is even wanted
+	case Literal:
+		if e.value == nil {
+			return "nil"
+		}
+		return fmt.Sprintf("%v", e.value)
+	case Unary:
+		return rpn(e.right) + fmt.Sprintf(" %v", e.operator.Lexeme)
+	}
+	return "ERROR: reached impossible branch in print function"
+}