Rename TypeAlloc to Pool

Minor refactor to clarify purpose of the pool.

src/mem/pooled.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include "../ds/bits.h"
+
+namespace snmalloc
+{
+  template<class T>
+  class Pooled
+  {
+  private:
+    template<class TT, class MemoryProvider>
+    friend class Pool;
+    template<class TT, Construction c>
+    friend class MPMCStack;
+
+    /// Used by the pool for chaining together entries when not in use.
+    std::atomic<T*> next = nullptr;
+    /// Used by the pool to keep the list of all entries ever created.
+    T* list_next;
+  };
+}