language: introduce the requirement to use return to return a value from a function

examples/fib.pork

@@ -1,6 +1,6 @@
 /* fibonacci sequence */
 func fib(n) {
-  if n < 2 {
+  return if n < 2 {
     n
   } else {
     fib(n - 1) + fib(n - 2)
@@ -8,6 +8,6 @@ func fib(n) {
 }
 
 export func main() {
-  let result = fib(30)
+  let result = fib(20)
   println(result)
 }

examples/gameoflife/SDL2.pork

@@ -15,11 +15,11 @@ export func SDL_Quit()
 export let SDL_WINDOW_ALLOW_HIGHDPI = 8192
 
 export let SDL_WINDOWPOS_UNDEFINED_MASK = 536805376
-export func SDL_WINDOWPOS_UNDEFINED_DISPLAY(x) { SDL_WINDOWPOS_UNDEFINED_MASK | x }
+export func SDL_WINDOWPOS_UNDEFINED_DISPLAY(x) { return SDL_WINDOWPOS_UNDEFINED_MASK | x }
 export let SDL_WINDOWPOS_UNDEFINED = SDL_WINDOWPOS_UNDEFINED_DISPLAY(0)
 
 export let SDL_WINDOWPOS_CENTERED_MASK = 805240832
-export func SDL_WINDOWPOS_CENTERED_DISPLAY(x) { SDL_WINDOWPOS_CENTERED_MASK | x }
+export func SDL_WINDOWPOS_CENTERED_DISPLAY(x) { return SDL_WINDOWPOS_CENTERED_MASK | x }
 export let SDL_WINDOWPOS_CENTERED = SDL_WINDOWPOS_CENTERED_DISPLAY(0)
 
 export func SDL_CreateWindow(title, x, y, w, h, flags)

examples/gameoflife/gameoflife.pork

@@ -56,15 +56,15 @@ func drawCells(renderer, cells, swap) {
 
 func createCellGrid() {
   let numCells = gridWidth * gridHeight
-  listInitWith(numCells, 0)
+  return listInitWith(numCells, 0)
 }
 
 func getCell(cells, swap, x, y) {
   if (x >= 0) and (y >= 0) and (x < gridWidth) and (y < gridHeight) {
     let mask = if swap { 2 } else { 1 }
-    (cells[x + y * gridWidth] & mask) != 0
+    return (cells[x + y * gridWidth] & mask) != 0
   } else {
-    false
+    return false
   }
 }
 
@@ -88,7 +88,7 @@ func countNeighbours(cells, swap, x, y) {
   if getCell(cells, swap, x - 1, y + 1) { count++ }
   if getCell(cells, swap, x - 1, y) { count++ }
   if getCell(cells, swap, x - 1, y - 1) { count++ }
-  count
+  return count
 }
 
 func gameOfLife(cells, swap) {

examples/numbers.pork

@@ -1,5 +1,4 @@
 export func main() {
   let pi = 3.141592653589793
   println(pi)
-
 }