refact: use DoricSingleton to hold all static or singleton objects

doric-iOS/Pod/Classes/Loader/DoricJSLoaderManager.h

@@ -25,8 +25,6 @@
 #import "DoricAsyncResult.h"
 
 @interface DoricJSLoaderManager : NSObject
-+ (instancetype)instance;
-
 - (void)addJSLoader:(id <DoricLoaderProtocol>)loader;
 
 - (DoricAsyncResult <NSString *> *)request:(NSString *)source;

doric-iOS/Pod/Classes/Loader/DoricJSLoaderManager.m

@@ -30,15 +30,6 @@ @interface DoricJSLoaderManager ()
 @end
 
 @implementation DoricJSLoaderManager
-+ (instancetype)instance {
-    static DoricJSLoaderManager *_instance;
-    static dispatch_once_t onceToken;
-    dispatch_once(&onceToken, ^{
-        _instance = [DoricJSLoaderManager new];
-    });
-    return _instance;
-}
-
 - (instancetype)init {
     if (self = [super init]) {
         _loaders = [[NSSet alloc] initWithArray:@[