Support if without else.

examples/fib.pork

@@ -5,4 +5,5 @@ fib = { n in
     then 1
   else fib(n - 1) + fib(n - 2)
 }
+
 main = { in fib(20) }

examples/if.pork

@@ -0,0 +1,7 @@
+check = { value in
+  if value then 50
+}
+
+main = { in
+  value = check(100)
+}

examples/syntax.pork

@@ -18,12 +18,17 @@ main = { in
   trueValue = true
   falseValue = false
 
+  check = { in
+    if true then false else true
+  }
+
   [
     calculateSimpleResult,
     calculateComplexResult,
     multiplyResult,
     list,
     trueValue,
-    falseValue
+    falseValue,
+    check()
   ]
 }