compiler: add support for channel operations

Support for channels is not complete. The following pieces are missing:

  * Channels with values bigger than int. An int in TinyGo can always
    contain at least a pointer, so pointers are okay to send.
  * Buffered channels.
  * The select statement.

src/runtime/scheduler.go

@@ -51,10 +51,11 @@ func makeGoroutine(*uint8) *uint8
 
 // State/promise of a task. Internally represented as:
 //
-//     {i8* next, i32/i64 data}
+//     {i8* next, i1 commaOk, i32/i64 data}
 type taskState struct {
-	next *coroutine
-	data uint
+	next    *coroutine
+	commaOk bool // 'comma-ok' flag for channel receive operation
+	data    uint
 }
 
 // Queues used by the scheduler.