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/override/malloc.cc

@@ -221,10 +221,10 @@ extern "C"
    * this function to the correct type.  This allows us to preserve some
    * semblance of ABI safety via a pure C API.
    */
-  SNMALLOC_EXPORT void* SNMALLOC_NAME_MANGLE(snmalloc_pagemap_global_get)(
+  SNMALLOC_EXPORT void* SNMALLOC_NAME_MANGLE(snmalloc_chunkmap_global_get)(
     PagemapConfig const** config)
   {
-    auto& pm = GlobalPagemap::pagemap();
+    auto& pm = GlobalChunkmap::pagemap();
     if (config)
     {
       *config = &ChunkmapPagemap::config;