optimize unary minus for constants

lib/ruby_algebra/parser.rb

@@ -183,7 +183,11 @@ module RubyAlgebra
       when :positive
         out_stack.push a
       when :negative
-        out_stack.push Multiplication.new(Constant.new(-1), a)
+        if a.type == :constant
+          out_stack.push Constant.new(-a.value)
+        else
+          out_stack.push Multiplication.new(Constant.new(-1), a)
+        end
       end
     end