NFC: Introduce Purpose parameter for PageMap wrappers

Presently, GlobalPagemap and ExternalPagemap discriminate only by type.  If it
ever happened that multiple PageMap consumers instantiated the same type using
these wrapper, they'd be conflated in the symbol table.  Therefore, add an
optional Purpose parameter that will be expanded into the symbols (but serves no
other purpose).

src/mem/chunkmap.h

@@ -83,7 +83,9 @@ namespace snmalloc
     FlatPagemap<SUPERSLAB_BITS, uint8_t>,
     Pagemap<SUPERSLAB_BITS, uint8_t, 0>>;
 
-  using GlobalChunkmap = GlobalPagemapTemplate<ChunkmapPagemap>;
+  struct ForChunkmap
+  {};
+  using GlobalChunkmap = GlobalPagemapTemplate<ChunkmapPagemap, ForChunkmap>;
 
   /**
    * Optionally exported function that accesses the global chunkmap pagemap

src/mem/pagemap.h

@@ -422,13 +422,19 @@ namespace snmalloc
    * the memory will show up as not owned by any allocator in the other
    * compilation unit) but will prevent the same memory being interpreted as
    * having two different types.
+   *
+   * Simiarly, perhaps two modules wish to instantiate *different* pagemaps
+   * of the *same* type.  Therefore, we add a `Purpose` parameter that can be
+   * used to pry symbols apart.  By default, the `Purpose` is just the type of
+   * the pagemap; that is, pagemaps default to discrimination solely by their
+   * type.
    */
-  template<typename T>
+  template<typename T, typename Purpose = T>
   class GlobalPagemapTemplate
   {
     /**
      * The global pagemap variable.  The name of this symbol will include the
-     * type of `T`.
+     * type of `T` and `U`.
      */
     inline static T global_pagemap;
 
@@ -448,8 +454,13 @@ namespace snmalloc
    * library (e.g.  your C standard library) uses snmalloc and you wish to use a
    * different configuration in your program or library, but wish to share a
    * pagemap so that either version can deallocate memory.
+   *
+   * The `Purpose` parameter is as with `GlobalPgemapTemplate`.
    */
-  template<typename T, void* (*raw_get)(const PagemapConfig**)>
+  template<
+    typename T,
+    void* (*raw_get)(const PagemapConfig**),
+    typename Purpose = T>
   class ExternalGlobalPagemapTemplate
   {
     /**