Introduce header layering (#503)

See src/snmalloc/README.md for an explanation of the layers.

Some other cleanups on the way:

Fine-grained stats support is now gone.

It's been broken for two years, it depends on iostream (which then
causes linker failures with libstdc++) and it's collecting the wrong
stats for the new design.  After discussion with @mjp41, it's better to
remove it and introduce new stats support later, rather than keep broken
code in the main branch.

Tracing was controlled with a preprocessor macro, now there's also a
CMake option.

src/test/func/external_pagemap/external_pagemap.cc

@@ -9,7 +9,7 @@ int main()
 }
 #else
 #  define SNMALLOC_EXPOSE_PAGEMAP 1
-#  include <override/malloc.cc>
+#  include <snmalloc/override/malloc.cc>
 
 using ExternalChunkmap =
   ExternalGlobalPagemapTemplate<ChunkmapPagemap, snmalloc_chunkmap_global_get>;