NFC: Introduce and switch to pal_zero

This wrapper will allow us to pass `AuthPtr<T,B> p` to zero() without needing to
write `p.unsafe_auth_ptr` to get to a `T*` inside.  Moreover, it will give us a
convenient point to assert that `B` is such that the pointer can be used to
manipulate the memory map (i.e. is not exported).

src/mem/alloc.h

@@ -1026,7 +1026,8 @@ namespace snmalloc
         void* p = remove_cache_friendly_offset(fl.take(entropy), sizeclass);
         if constexpr (zero_mem == YesZero)
         {
-          MemoryProvider::Pal::zero(p, sizeclass_to_size(sizeclass));
+          pal_zero<typename MemoryProvider::Pal>(
+            p, sizeclass_to_size(sizeclass));
         }
         return p;
       }
@@ -1138,7 +1139,7 @@ namespace snmalloc
 
       if constexpr (zero_mem == YesZero)
       {
-        MemoryProvider::Pal::zero(p, sizeclass_to_size(sizeclass));
+        pal_zero<typename MemoryProvider::Pal>(p, sizeclass_to_size(sizeclass));
       }
       return p;
     }

src/mem/largealloc.h

@@ -319,7 +319,7 @@ namespace snmalloc
           // The first page is already in "use" for the stack element,
           // this will need zeroing for a YesZero call.
           if constexpr (zero_mem == YesZero)
-            MemoryProvider::Pal::template zero<true>(p, OS_PAGE_SIZE);
+            pal_zero<typename MemoryProvider::Pal, true>(p, OS_PAGE_SIZE);
 
           // Notify we are using the rest of the allocation.
           // Passing zero_mem ensures the PAL provides zeroed pages if
@@ -331,7 +331,7 @@ namespace snmalloc
         {
           // This is a superslab that has not been decommitted.
           if constexpr (zero_mem == YesZero)
-            MemoryProvider::Pal::template zero<true>(
+            pal_zero<typename MemoryProvider::Pal, true>(
               p, bits::align_up(size, OS_PAGE_SIZE));
           else
             UNUSED(size);

src/mem/mediumslab.h

@@ -94,7 +94,7 @@ namespace snmalloc
       self->free--;
 
       if constexpr (zero_mem == YesZero)
-        PAL::zero(p, size);
+        pal_zero<PAL>(p, size);
       else
         UNUSED(size);
 

src/mem/metaslab.h

@@ -181,9 +181,9 @@ namespace snmalloc
       if constexpr (zero_mem == YesZero)
       {
         if (rsize < PAGE_ALIGNED_SIZE)
-          PAL::zero(p, rsize);
+          pal_zero<PAL>(p, rsize);
         else
-          PAL::template zero<true>(p, rsize);
+          pal_zero<PAL, true>(p, rsize);
       }
       else
       {

src/pal/pal.h

@@ -69,6 +69,16 @@ namespace snmalloc
   // Used to keep Superslab metadata committed.
   static constexpr size_t OS_PAGE_SIZE = Pal::page_size;
 
+  /**
+   * A centralized, inlinable wrapper around PAL::zero.  This will matter more
+   * when we introduce AuthPtr-s.
+   */
+  template<typename PAL, bool page_aligned = false>
+  static SNMALLOC_FAST_PATH void pal_zero(void* p, size_t sz)
+  {
+    PAL::template zero<page_aligned>(p, sz);
+  }
+
   static_assert(
     bits::is_pow2(OS_PAGE_SIZE), "OS_PAGE_SIZE must be a power of two");
   static_assert(