NFC: Add an AAL Concept, too

While here, pull out some constants to their own header.  Eventually we'll
want to match on AalFeatures in the AAL Concept.

src/aal/aal_concept.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#ifdef __cpp_concepts
+#  include "../ds/concept.h"
+#  include "aal_consts.h"
+
+#  include <cstdint>
+#  include <utility>
+
+namespace snmalloc
+{
+  /**
+   * AALs must advertise the bit vector of supported features, their name,
+   * 
+   */
+  template<typename AAL>
+  concept ConceptAAL_static_members = requires()
+  {
+    typename std::integral_constant<uint64_t, AAL::aal_features>;
+    typename std::integral_constant<int, AAL::aal_name>;
+  };
+
+  /**
+   * AALs provide a prefetch operation.
+   */
+  template<typename AAL>
+  concept ConceptAAL_prefetch = requires(void *ptr)
+  {
+    { AAL::prefetch(ptr) } noexcept -> ConceptSame<void>;
+  };
+
+  /**
+   * AALs provide a notion of high-precision timing.
+   */
+  template<typename AAL>
+  concept ConceptAAL_tick = requires()
+  {
+    { AAL::tick() } noexcept -> ConceptSame<uint64_t>;
+  };
+
+  template<typename AAL>
+  concept ConceptAAL =
+    ConceptAAL_static_members<AAL> &&
+    ConceptAAL_prefetch<AAL> &&
+    ConceptAAL_tick<AAL>;
+
+} // namespace snmalloc
+#endif