Snmalloc2 API cleanups for sandbox use. (#359)

This is the set of changes required for snmalloc2 to be usable by the
process sandboxing code and incorporates some API changes that reduce
the amount of code required to embed snmalloc.  Highlights:

 - Merge the config and back-end classes.
 - Everything in config is now global (all methods are static)
 - The GlobalState class is gone (all global state is managed by global
   methods on the config class)
 - LocalState is now a member of the config class, all methods are
   instance methods.
 - Not every configuration needs to use the lazy initialisation hooks.
   They now need to be provided only if they are used.  If the
   configuration does not provide an `ensure_init` method, it is not
   called.  If it does not provide an `is_initialised` method then the
   global initialisation state is not checked.
 - There is now an `snmalloc::Options` class that default initialises
   itself to the default behaviour.  Every configuration must provide a
   `constexpr` instance of this class.  Each flag can be separately
   overridden and new flags can be added without breaking any existing
   API consumers.

The config classes are moved into the backend directory.

src/test/perf/external_pointer/externalpointer.cc

@@ -47,7 +47,7 @@ namespace test
       alloc.dealloc(objects[i]);
     }
 
-    snmalloc::debug_check_empty(Globals::get_handle());
+    snmalloc::debug_check_empty<Globals>();
   }
 
   void test_external_pointer(xoroshiro::p128r64& r)