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/contention/contention.cc

@@ -5,7 +5,7 @@
 
 #include <iomanip>
 #include <iostream>
-#include <snmalloc.h>
+#include <snmalloc/snmalloc.h>
 #include <thread>
 #include <vector>
 

src/test/perf/external_pointer/externalpointer.cc

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

src/test/perf/low_memory/low-memory.cc

@@ -1,5 +1,5 @@
 #include <iostream>
-#include <snmalloc.h>
+#include <snmalloc/snmalloc.h>
 #include <test/opt.h>
 #include <test/setup.h>
 #include <unordered_set>
@@ -164,4 +164,4 @@ int main(int argc, char** argv)
   //   std::cout << "Release test only." << std::endl;
   // #endif
   return 0;
-}
+}

src/test/perf/memcpy/memcpy.cc

@@ -1,4 +1,4 @@
-#include "mem/memcpy.h"
+#include "snmalloc/global/memcpy.h"
 
 #include <test/measuretime.h>
 #include <test/opt.h>

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>