Release address space on thread exit (#1188)

The destructor for arena now munmap()s any mapped address space. This is
aimed at threads that may exit while the address space remains active
for other threads.

include/runtime/arena.h

@@ -4,6 +4,7 @@
 #include <algorithm>
 #include <cstddef>
 #include <cstdint>
+#include <sys/mman.h>
 #include <sys/types.h>
 #include <utility>
 
@@ -22,6 +23,12 @@ class arena {
     initialize_semispace();
   }
 
+  ~arena() {
+    munmap(current_addr_ptr, HYPERBLOCK_SIZE);
+    if (collection_addr_ptr)
+      munmap(collection_addr_ptr, HYPERBLOCK_SIZE);
+  }
+
   char *evacuate(char *scan_ptr);
 
   // Allocates the requested number of bytes as a contiguous region and returns a