Add C++ concept for PAL

This will not be used unless the C++ standard version is raised to 20.  As
concepts and C++20 more generally are quite new, this does not do so.
Nevertheless, the use of concepts can improve the local development experience
as type mismatches are discovered earlier (at template invocation rather than
only during expansion).

src/ds/concept.h

@@ -0,0 +1,40 @@
+#pragma once
+
+/**
+ * C++20 concepts are referenced as if they were types in declarations within
+ * template parameters (e.g. "template<FooConcept Foo> ...").  That is, they
+ * take the place of the "typename"/"class" keyword on template parameters.
+ * If the compiler understands concepts, this macro expands as its argument;
+ * otherwise, it expands to the keyword "typename", so snmalloc templates that
+ * use concept-qualified parameters should use this to remain compatible across
+ * C++ versions: "template<SNMALLOC_CONCEPT(FooConcept) Foo>"
+ */
+#ifdef __cpp_concepts
+#  define SNMALLOC_CONCEPT(c) c
+#else
+#  define SNMALLOC_CONCEPT(c) typename
+#endif
+
+#ifdef __cpp_concepts
+namespace snmalloc
+{
+  /**
+   * C++20 concepts are more than just new syntax; there's a new support
+   * library specified as well.  As C++20 is quite new, however, there are some
+   * environments, notably Clang, that understand the syntax but do not yet
+   * offer the library.  Fortunately, alternate pronouciations are possible.
+   */
+#  ifdef _cpp_lib_concepts
+  /**
+   * ConceptSame<T,U> is true if T and U are the same type and false otherwise.
+   * When specifying a concept, use ConceptSame<U> to indicate that an
+   * expression must evaluate precisely to the type U.
+   */
+  template<typename T, typename U>
+  concept ConceptSame = std::same_as<T, U>;
+#  else
+  template<typename T, typename U>
+  concept ConceptSame = std::is_same<T, U>::value;
+#  endif
+} // namespace snmalloc
+#endif