NFC: Further disentangle chunkmap and pagemap

- Make GlobalPagemapTemplate and ExternalGlobalPagemap generic in the type of
  the pagemap they're encapsulating.

  We're going to want to use these for other kinds of pagemaps in the near
  future.

- Rename snmalloc_pagemap_global_get to snmalloc_chunkmap_global_get.

- Rename GlobalPagemap to GlobalChunkmap.

src/test/func/external_pagemap/external_pagemap.cc

@@ -10,10 +10,13 @@ int main()
 #  define SNMALLOC_EXPOSE_PAGEMAP 1
 #  include <override/malloc.cc>
 
+using ExternalChunkmap =
+  ExternalGlobalPagemapTemplate<ChunkmapPagemap, snmalloc_chunkmap_global_get>;
+
 int main()
 {
-  auto& p = ExternalGlobalPagemap::pagemap();
-  auto& global = GlobalPagemap::pagemap();
+  auto& p = ExternalChunkmap::pagemap();
+  auto& global = GlobalChunkmap::pagemap();
   SNMALLOC_CHECK(&p == &global);
   // Get a valid heap address
   uintptr_t addr = reinterpret_cast<uintptr_t>(malloc(42));

src/test/func/two_alloc_types/main.cc

@@ -24,9 +24,9 @@ extern "C" void* enclave_malloc(size_t);
 extern "C" void enclave_free(void*);
 
 extern "C" void*
-enclave_snmalloc_pagemap_global_get(snmalloc::PagemapConfig const**);
+enclave_snmalloc_chunkmap_global_get(snmalloc::PagemapConfig const**);
 extern "C" void*
-host_snmalloc_pagemap_global_get(snmalloc::PagemapConfig const**);
+host_snmalloc_chunkmap_global_get(snmalloc::PagemapConfig const**);
 
 using namespace snmalloc;
 int main()
@@ -47,8 +47,8 @@ int main()
 
   // Call these functions to trigger asserts if the cast-to-self doesn't work.
   const PagemapConfig* c;
-  enclave_snmalloc_pagemap_global_get(&c);
-  host_snmalloc_pagemap_global_get(&c);
+  enclave_snmalloc_chunkmap_global_get(&c);
+  host_snmalloc_chunkmap_global_get(&c);
 
   auto a = host_malloc(128);
   auto b = enclave_malloc(128);