Improved ThreadAlloc::get API

Made the API so that get always returns an initialised Alloc*.  Added
new fast path that doesn't perform checking, but can lead to very slow
behaviour if called and reused.

src/test/perf/contention/contention.cc

@@ -75,7 +75,7 @@ size_t swapcount;
 
 void test_tasks_f(size_t id)
 {
-  Alloc*& a = ThreadAlloc::get();
+  Alloc* a = ThreadAlloc::get();
   xoroshiro::p128r32 r(id + 5000);
 
   for (size_t n = 0; n < swapcount; n++)
@@ -100,7 +100,7 @@ void test_tasks_f(size_t id)
 
 void test_tasks(size_t num_tasks, size_t count, size_t size)
 {
-  Alloc*& a = ThreadAlloc::get();
+  Alloc* a = ThreadAlloc::get();
 
   contention = new std::atomic<size_t*>[size];
   xoroshiro::p128r32 r;

src/test/perf/external_pointer/externalpointer.cc

@@ -49,7 +49,7 @@ namespace test
 
   void test_external_pointer(xoroshiro::p128r64& r)
   {
-    auto& alloc = ThreadAlloc::get();
+    auto alloc = ThreadAlloc::get();
 #ifdef NDEBUG
     static constexpr size_t iterations = 10000000;
 #else
@@ -79,9 +79,6 @@ namespace test
 int main(int, char**)
 {
   xoroshiro::p128r64 r;
-  // Force a per-thread allocator to actually exist.
-  void* p = ThreadAlloc::get()->alloc(16);
-  ThreadAlloc::get()->dealloc(p);
 #if NDEBUG
   size_t nn = 30;
 #else

src/test/perf/singlethread/singlethread.cc

@@ -7,7 +7,7 @@ using namespace snmalloc;
 template<ZeroMem zero_mem>
 void test_alloc_dealloc(size_t count, size_t size, bool write)
 {
-  auto*& alloc = ThreadAlloc::get();
+  auto* alloc = ThreadAlloc::get();
 
   DO_TIME(
     "Count: " << std::setw(6) << count << ", Size: " << std::setw(6) << size