all: rename [ref_only] -> [heap] (#8718)

vlib/sync/sync_default.c.v

@@ -28,12 +28,12 @@ fn C.sem_timedwait(voidptr, voidptr) int
 fn C.sem_destroy(voidptr) int
 
 // [init_with=new_mutex] // TODO: implement support for this struct attribute, and disallow Mutex{} from outside the sync.new_mutex() function.
-[ref_only]
+[heap]
 pub struct Mutex {
 	mutex C.pthread_mutex_t
 }
 
-[ref_only]
+[heap]
 pub struct RwMutex {
 	mutex C.pthread_rwlock_t
 }
@@ -42,7 +42,7 @@ struct RwMutexAttr {
 	attr C.pthread_rwlockattr_t
 }
 
-[ref_only]
+[heap]
 struct Semaphore {
 	sem C.sem_t
 }

vlib/sync/sync_macos.c.v

@@ -31,12 +31,12 @@ fn C.pthread_cond_timedwait(voidptr, voidptr, voidptr) int
 fn C.pthread_cond_destroy(voidptr) int
 
 // [init_with=new_mutex] // TODO: implement support for this struct attribute, and disallow Mutex{} from outside the sync.new_mutex() function.
-[ref_only]
+[heap]
 pub struct Mutex {
 	mutex C.pthread_mutex_t
 }
 
-[ref_only]
+[heap]
 pub struct RwMutex {
 	mutex C.pthread_rwlock_t
 }
@@ -51,7 +51,7 @@ struct CondAttr {
 
 /* MacOSX has no unnamed semaphores and no `timed_wait()` at all
    so we emulate the behaviour with other devices */
-[ref_only]
+[heap]
 struct Semaphore {
 	mtx C.pthread_mutex_t
 	cond C.pthread_cond_t

vlib/sync/sync_windows.c.v

@@ -22,19 +22,19 @@ type SHANDLE = voidptr
 //[init_with=new_mutex] // TODO: implement support for this struct attribute, and disallow Mutex{} from outside the sync.new_mutex() function.
 
 // `SRWLOCK` is much more performant that `Mutex` on Windows, so use that in both cases since we don't want to share with other processes
-[ref_only]
+[heap]
 pub struct Mutex {
 mut:
 	mx C.SRWLOCK    // mutex handle
 }
 
-[ref_only]
+[heap]
 pub struct RwMutex {
 mut:
 	mx C.SRWLOCK    // mutex handle
 }
 
-[ref_only]
+[heap]
 struct Semaphore {
 	mtx C.SRWLOCK
 	cond C.CONDITION_VARIABLE

vlib/sync/waitgroup.v

@@ -18,7 +18,7 @@ fn C.atomic_fetch_add_u32(voidptr, u32) u32
 // `wg.done()` when finished 
 //
 // [init_with=new_waitgroup] // TODO: implement support for init_with struct attribute, and disallow WaitGroup{} from outside the sync.new_waitgroup() function.
-[ref_only]
+[heap]
 struct WaitGroup {
 mut:
 	task_count u32 // current task count - reading/writing should be atomic