Use a accumulator-character-token in the tokenizer to reduce the initialization overhead

Initializing character tokens, and the NSString objects they use, results in a relatively high overhead. The tokenizer
now accumulated all successive characters in on accumulator token until a non-character token is emitted.

This change reduces the execution time of the performance test on the local machine by 48% (reference to baseline)

HTMLKitTests/HTMLKitTokenizerPerformance.m

@@ -26,11 +26,7 @@
 
 	[self measureBlock:^{
 		HTMLTokenizer *tokenizer = [[HTMLTokenizer alloc] initWithString:string];
-
-		id token = nil;
-		do {
-			token = [tokenizer nextObject];
-		} while (token != nil);
+		[tokenizer allObjects];
 	}];
 }