Prevent TracePoint objects from being GC'd

By just stuffing these into the Allocations module as a constant we can
prevent them from being GC'd.

ext/liballocations/liballocations.c

@@ -126,12 +126,6 @@ VALUE allocations_start(VALUE self) {
     rb_mutex_unlock(mutex);
-    allocation_tracer = rb_tracepoint_new(Qnil, RUBY_INTERNAL_EVENT_NEWOBJ,
-                                          newobj_callback, NULL);
-
-    free_tracer = rb_tracepoint_new(Qnil, RUBY_INTERNAL_EVENT_FREEOBJ,
-                                    freeobj_callback, NULL);
-
     rb_tracepoint_enable(allocation_tracer);
     rb_tracepoint_enable(free_tracer);
@@ -153,10 +147,8 @@ VALUE allocations_stop(VALUE self) {
         return Qnil;
     }
-    if ( allocation_tracer && free_tracer ) {
-        rb_tracepoint_disable(allocation_tracer);
-        rb_tracepoint_disable(free_tracer);
-    }
+    rb_tracepoint_disable(allocation_tracer);
+    rb_tracepoint_disable(free_tracer);
     if ( object_counts ) {
         st_free_table(object_counts);
@@ -194,6 +186,12 @@ VALUE allocations_enabled_p(VALUE self) {
 void Init_liballocations() {
     VALUE mAllocations = rb_define_module_under(rb_cObject, "Allocations");
+    allocation_tracer = rb_tracepoint_new(Qnil, RUBY_INTERNAL_EVENT_NEWOBJ,
+                                          newobj_callback, NULL);
+
+    free_tracer = rb_tracepoint_new(Qnil, RUBY_INTERNAL_EVENT_FREEOBJ,
+                                    freeobj_callback, NULL);
+
     mutex = rb_mutex_new();
     id_enabled = rb_intern("enabled");
@@ -204,4 +202,6 @@ void Init_liballocations() {
     rb_define_singleton_method(mAllocations, "enabled?", allocations_enabled_p, 0);
     rb_define_const(mAllocations, "MUTEX", mutex);
+    rb_define_const(mAllocations, "ALLOCATION_TRACER", allocation_tracer);
+    rb_define_const(mAllocations, "FREE_TRACER", free_tracer);
 }