AK: Move StringImpl::operator== implementation into StringImpl

Author: https://github.com/nico
Commit: cc765e14ca
Pull-request: https://github.com/SerenityOS/serenity/pull/3693
Reviewed-by: https://github.com/awesomekling

AK/String.cpp

@@ -60,10 +60,7 @@ bool String::operator==(const String& other) const
     if (!other.m_impl)
         return false;
 
-    if (length() != other.length())
-        return false;
-
-    return !memcmp(characters(), other.characters(), length());
+    return *m_impl == *other.m_impl;
 }
 
 bool String::operator==(const StringView& other) const