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/perf/singlethread/singlethread.cc

@@ -1,4 +1,4 @@
-#include <snmalloc.h>
+#include <snmalloc/snmalloc.h>
 #include <test/measuretime.h>
 #include <test/setup.h>
 #include <unordered_set>