AK: Add String::number() for creating a String from a number.

Instead of manually doing String::format("%d"/"%u") everywhere, let's have
a String API for this. It's just a wrapper around format() for now, but it
could be made more efficient in the future.

AK/AKString.h

@@ -181,6 +181,8 @@ public:
     }
 
     static String format(const char*, ...);
+    static String number(unsigned);
+    static String number(int);
 
     StringView view() const { return { characters(), length() }; }
 

AK/String.cpp

@@ -170,6 +170,16 @@ unsigned String::to_uint(bool& ok) const
     return value;
 }
 
+String String::number(unsigned value)
+{
+    return String::format("%u", value);
+}
+
+String String::number(int value)
+{
+    return String::format("%d", value);
+}
+
 String String::format(const char* fmt, ...)
 {
     StringBuilder builder;

AK/Tests/TestQueue.cpp

@@ -28,7 +28,7 @@ int main()
     EXPECT(strings.is_empty());
 
     for (int i = 0; i < 10000; ++i) {
-        strings.enqueue(String::format("%d", i));
+        strings.enqueue(String::number(i));
         EXPECT_EQ(strings.size(), i + 1);
     }