alloc: de-static external_pointer

Like alloc_size, this will require amplification internally.

This patch also restores performance to the status quo ante; Clang can once
again see enough to generate the same code as it did before de-static-ing
alloc_size.

src/test/func/memory/memory.cc

@@ -241,8 +241,8 @@ void test_external_pointer()
     for (size_t offset = 0; offset < size; offset += 17)
     {
       void* p2 = pointer_offset(p1, offset);
-      void* p3 = Alloc::external_pointer(p2);
-      void* p4 = Alloc::external_pointer<End>(p2);
+      void* p3 = alloc->external_pointer(p2);
+      void* p4 = alloc->external_pointer<End>(p2);
       UNUSED(p3);
       UNUSED(p4);
       SNMALLOC_CHECK(p1 == p3);
@@ -257,7 +257,8 @@ void test_external_pointer()
 
 void check_offset(void* base, void* interior)
 {
-  void* calced_base = Alloc::external_pointer((void*)interior);
+  auto alloc = ThreadAlloc::get();
+  void* calced_base = alloc->external_pointer((void*)interior);
   if (calced_base != (void*)base)
     abort();
 }
@@ -335,7 +336,7 @@ void test_external_pointer_dealloc_bug()
 
   for (size_t i = 0; i < count; i++)
   {
-    Alloc::external_pointer(allocs[i]);
+    alloc->external_pointer(allocs[i]);
   }
 
   alloc->dealloc(allocs[0]);
@@ -348,7 +349,7 @@ void test_alloc_16M()
   const size_t size = 16'000'000;
 
   void* p1 = alloc->alloc(size);
-  SNMALLOC_CHECK(alloc->alloc_size(Alloc::external_pointer(p1)) >= size);
+  SNMALLOC_CHECK(alloc->alloc_size(alloc->external_pointer(p1)) >= size);
   alloc->dealloc(p1);
 }
 
@@ -359,7 +360,7 @@ void test_calloc_16M()
   const size_t size = 16'000'000;
 
   void* p1 = alloc->alloc<YesZero>(size);
-  SNMALLOC_CHECK(alloc->alloc_size(Alloc::external_pointer(p1)) >= size);
+  SNMALLOC_CHECK(alloc->alloc_size(alloc->external_pointer(p1)) >= size);
   alloc->dealloc(p1);
 }
 
@@ -373,7 +374,7 @@ void test_calloc_large_bug()
   const size_t size = (SUPERSLAB_SIZE << 3) - 7;
 
   void* p1 = alloc->alloc<YesZero>(size);
-  SNMALLOC_CHECK(alloc->alloc_size(Alloc::external_pointer(p1)) >= size);
+  SNMALLOC_CHECK(alloc->alloc_size(alloc->external_pointer(p1)) >= size);
   alloc->dealloc(p1);
 }