mm: refactor page allocator

This is hopefully the last time in a while that
something in the mm subsystem needs a refactor
this large.  There are two main changes:
- The page frame allocator returns a vm_page_t
  rather than a virtual address.
- Data for the slab allocator is now stored in
  struct vm_page, which means there is no overhead
  in the slab itself so the space is used in a
  more efficient manner.

lib/c/include/string.h

@@ -2,6 +2,8 @@
 
 #pragma once
 
+#include <arch/string.h>
+
 #include <gay/cdefs.h>
 #include <gay/types.h>
 
@@ -71,6 +73,7 @@ void *memcpy(void *__restrict dest, const void *__restrict src, usize n);
  */
 __pure int memcmp(const void *s1, const void *s2, usize n);
 
+#ifndef __HAVE_ARCH_MEMSET
 /**
  * @brief Starting from `ptr`, fill `n` bytes with the constant byte `c`.
  *
@@ -80,6 +83,28 @@ __pure int memcmp(const void *s1, const void *s2, usize n);
  * @returns A pointer to `ptr`
  */
 void *memset(void *ptr, int c, usize n);
+#endif
+
+#if _GAY_SOURCE >= 202109L
+#ifndef __HAVE_ARCH_MEMSET16
+void *memset16(u16 *dest, u16 c, usize nbyte);
+#endif
+#ifndef __HAVE_ARCH_MEMSET32
+void *memset32(u32 *dest, u32 c, usize nbyte);
+#endif
+#ifndef __HAVE_ARCH_MEMSET64
+void *memset64(u64 *dest, u64 c, usize nbyte);
+#endif
+
+#include <limits.h>
+#if LONG_BIT == 32
+#define memsetl memset32
+#elif LONG_BIT == 64
+#define memsetl memset64
+#else
+#error "Unsupported sizeof(long)"
+#endif
+#endif
 
 /**
  * @brief Copy a memory area.