Add Axiom contract group that is never evaluated (lowers to nothing)

For now the condition just has to be grammatical. We can even relax that if we want... I don't currently know of a reason we would want to allow an arbitrary token sequence there, but if there is that would be easy to allow

Author: hsutter

regression-tests/pure2-contracts.cpp2

@@ -0,0 +1,5 @@
+main: () = {
+    assert( 1 != 2, "ack, arithmetic is buggy" );
+    assert<Type>( typeid(int) != typeid(double), "ack, C types are broken" );
+    assert<Axiom>( any-grammatical.kind(of, nonsense * here) is "all ignored" );
+}

regression-tests/test-results/clang-12/pure2-contracts.cpp.execution

@@ -0,0 +1 @@
+pure2-contracts.cpp

regression-tests/test-results/clang-12/pure2-contracts.cpp.output

@@ -0,0 +1,26 @@
+
+#define CPP2_IMPORT_STD          Yes
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+#line 1 "pure2-contracts.cpp2"
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "pure2-contracts.cpp2"
+auto main() -> int;
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "pure2-contracts.cpp2"
+auto main() -> int{
+#line 2 "pure2-contracts.cpp2"
+    cpp2::Default.expects(1 != 2, "ack, arithmetic is buggy");
+    cpp2::Type.expects(typeid(int) != typeid(double), "ack, C types are broken");
+
+}
+