AK: Move memory stuff (fast memcpy, etc) to a separate header

Move the "fast memcpy" stuff out of StdLibExtras.h and into Memory.h.
This will break a ton of things that were relying on StdLibExtras.h
to include a bunch of other headers. Fix will follow immediately after.

This makes it possible to include StdLibExtras.h from Types.h, which is
the main point of this exercise.

AK/Memory.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <AK/Types.h>
+
+#if defined(KERNEL) || defined(BOOTSTRAPPER)
+#    include <LibBareMetal/StdLib.h>
+#else
+#    include <stdlib.h>
+#    include <string.h>
+#endif
+
+#if defined(__serenity__) && !defined(KERNEL) && !defined(BOOTSTRAPPER)
+extern "C" void* mmx_memcpy(void* to, const void* from, size_t);
+#endif
+
+[[gnu::always_inline]] inline void fast_u32_copy(u32* dest, const u32* src, size_t count)
+{
+#if defined(__serenity__) && !defined(KERNEL) && !defined(BOOTSTRAPPER)
+    if (count >= 256) {
+        mmx_memcpy(dest, src, count * sizeof(count));
+        return;
+    }
+#endif
+    asm volatile(
+        "rep movsl\n"
+        : "=S"(src), "=D"(dest), "=c"(count)
+        : "S"(src), "D"(dest), "c"(count)
+        : "memory");
+}
+
+[[gnu::always_inline]] inline void fast_u32_fill(u32* dest, u32 value, size_t count)
+{
+    asm volatile(
+        "rep stosl\n"
+        : "=D"(dest), "=c"(count)
+        : "D"(dest), "c"(count), "a"(value)
+        : "memory");
+}