Change if function stack ordering, add some basic math ops to Int, add factorial example

* `if` function stack ordering is changed to use condition, if-false,
  if-true (bottom to top). This is so we can pass a value on top of the
  stack directly to a new `if` in a condition, so we don't have to store
  a new value in a new variable.
* Add __splat__ and __minus__ functions to Int
* Add * and - builtins, which call the __splat__ and __minus__ functions
  of the top stack item, respectively.
* Add factorial example because now factorials are possible with this
  skeleton implementation, albeit a little hacky with the recursion.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

examples/branch.sy

@@ -1,7 +1,7 @@
-[ "fail - 0 is not truthy" println! ] [ "OK" println! ] 0 if!
-[ "OK" println! ] [ "fail - 1 is truthy" println! ] 1 if!
-[ "OK" println! ] [ "fail - 1.1 is truthy" println! ] 1.1 if!
-[ "OK" println! ] [ "fail - 0.1 is truthy" println! ] 0.1 if!
-[ "OK" println! ] [ "fail - -0.1 is truthy" println! ] -0.1 if!
-[ "fail - -0.0 is not truthy" println! ] [ "OK" println! ] -0.0 if!
-[ "fail - 0.0 is not truthy" println! ] [ "OK" println! ] 0.0 if!
+0 [ "fail - 0 is not truthy" println! ] [ "OK" println! ] if!
+1 [ "OK" println! ] [ "fail - 1 is truthy" println! ] if!
+1.1 [ "OK" println! ] [ "fail - 1.1 is truthy" println! ] if!
+0.1 [ "OK" println! ] [ "fail - 0.1 is truthy" println! ] if!
+-0.1 [ "OK" println! ] [ "fail - -0.1 is truthy" println! ] if!
+-0.0 [ "fail - -0.0 is not truthy" println! ] [ "OK" println! ] if!
+0.0 [ "fail - 0.0 is not truthy" println! ] [ "OK" println! ] if!

examples/factorial.sy

@@ -0,0 +1,28 @@
+[ :x x x ] :dup
+
+0 :factorial
+[
+    dup!
+    [dup! 1 -! factorial! *!]
+    [:_ 1]
+    if!
+]
+:factorial
+
+[
+    dup! print!
+    "! = " print!
+    factorial!
+    println!
+]
+:do-factorial
+
+1 do-factorial!
+2 do-factorial!
+3 do-factorial!
+4 do-factorial!
+5 do-factorial!
+6 do-factorial!
+7 do-factorial!
+8 do-factorial!
+9 do-factorial!