AK+Kernel+LibELF: Remove the need for IteratorDecision::Continue

By constraining two implementations, the compiler will select the best
fitting one. All this will require is duplicating the implementation and
simplifying for the `void` case.

This constraining also informs both the caller and compiler by passing
the callback parameter types as part of the constraint
(e.g.: `IterationFunction<int>`).

Some `for_each` functions in LibELF only take functions which return
`void`. This is a minimal correctness check, as it removes one way for a
function to incompletely do something.

There seems to be a possible idiom where inside a lambda, a `return;` is
the same as `continue;` in a for-loop.

AK/Concepts.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/IterationDecision.h>
 #include <AK/StdLibExtras.h>
 
 namespace AK::Concepts {
@@ -27,6 +28,27 @@ concept Signed = IsSigned<T>;
 template<typename T>
 concept Unsigned = IsUnsigned<T>;
 
+template<typename T, typename U>
+concept SameAs = IsSame<T, U>;
+
+template<typename Func, typename... Args>
+concept VoidFunction = requires(Func func, Args... args)
+{
+    {
+        func(args...)
+    }
+    ->SameAs<void>;
+};
+
+template<typename Func, typename... Args>
+concept IteratorFunction = requires(Func func, Args... args)
+{
+    {
+        func(args...)
+    }
+    ->SameAs<IterationDecision>;
+};
+
 #endif
 
 }
@@ -36,7 +58,9 @@ concept Unsigned = IsUnsigned<T>;
 using AK::Concepts::Arithmetic;
 using AK::Concepts::FloatingPoint;
 using AK::Concepts::Integral;
+using AK::Concepts::IteratorFunction;
 using AK::Concepts::Signed;
 using AK::Concepts::Unsigned;
+using AK::Concepts::VoidFunction;
 
 #endif

AK/InlineLinkedList.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Assertions.h>
+#include <AK/Concepts.h>
 #include <AK/Types.h>
 
 namespace AK {
@@ -112,7 +113,7 @@ public:
         return false;
     }
 
-    template<typename F>
+    template<IteratorFunction<T&> F>
     IterationDecision for_each(F func) const
     {
         for (T* node = m_head; node; node = node->next()) {
@@ -123,6 +124,13 @@ public:
         return IterationDecision::Continue;
     }
 
+    template<VoidFunction<T&> F>
+    void for_each(F func) const
+    {
+        for (T* node = m_head; node; node = node->next())
+            func(*node);
+    }
+
     using Iterator = InlineLinkedListIterator<T>;
     friend Iterator;
     Iterator begin() { return Iterator(m_head); }