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/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