Don't allow fibers as map keys.

I hacked in support for it for a misguided reason (trying to fake
"thread-local storage") and ended up not using it for that anyway.

src/vm/wren_primitive.c

@@ -54,13 +54,13 @@ bool validateInt(WrenVM* vm, Value arg, const char* argName)
 
 bool validateKey(WrenVM* vm, Value arg)
 {
-  if (IS_BOOL(arg) || IS_CLASS(arg) || IS_FIBER(arg) || IS_NULL(arg) ||
+  if (IS_BOOL(arg) || IS_CLASS(arg) || IS_NULL(arg) ||
       IS_NUM(arg) || IS_RANGE(arg) || IS_STRING(arg))
   {
     return true;
   }
 
-  vm->fiber->error = CONST_STRING(vm, "Key must be a value type or fiber.");
+  vm->fiber->error = CONST_STRING(vm, "Key must be a value type.");
   return false;
 }
 

src/vm/wren_value.c

@@ -155,7 +155,6 @@ ObjFiber* wrenNewFiber(WrenVM* vm, Obj* fn)
   
   ObjFiber* fiber = ALLOCATE(vm, ObjFiber);
   initObj(vm, &fiber->obj, OBJ_FIBER, vm->fiberClass);
-  fiber->id = vm->nextFiberId++;
   fiber->frames = frames;
   fiber->frameCapacity = INITIAL_CALL_FRAMES;
   fiber->stack = stack;
@@ -344,9 +343,6 @@ static uint32_t hashObject(Obj* object)
       // Classes just use their name.
       return hashObject((Obj*)((ObjClass*)object)->name);
 
-    case OBJ_FIBER:
-      return ((ObjFiber*)object)->id;
-
     case OBJ_RANGE:
     {
       ObjRange* range = (ObjRange*)object;

src/vm/wren_value.h

@@ -242,10 +242,6 @@ typedef struct sObjFiber
   // error object. Otherwise, it will be null.
   Value error;
 
-  // A unique-ish numeric ID for the fiber. Lets fibers be used as map keys.
-  // Unique-ish since IDs may overflow and wrap around.
-  uint16_t id;
-
   // This will be true if the caller that called this fiber did so using "try".
   // In that case, if this fiber fails with an error, the error will be given
   // to the caller.

src/vm/wren_vm.h

@@ -50,11 +50,6 @@ struct WrenVM
   // for the module.
   ObjMap* modules;
 
-  // The ID that will be assigned to the next fiber that is allocated. Fibers
-  // are given unique-ish (not completely unique since this can overflow) IDs
-  // so that they can be used as map keys.
-  uint16_t nextFiberId;
-
   // Memory management data:
 
   // The number of bytes that are known to be currently allocated. Includes all