Add basic _requires-clause_ support

Should merge user-written _requires-clause_ expressions with any generated from `forward` parameters

Author: hsutter

regression-tests/pure2-requires-clauses.cpp2

@@ -0,0 +1,21 @@
+
+X:  <T: type, U: type>
+    type
+    requires std::is_same_v<T, int>
+              && std::is_same_v<U, int>
+= {
+    operator=: (out this) = { }
+}
+
+f:  <T: type, U: type>
+    (forward a: int, forward b: int) -> int
+    requires std::is_same_v<T, int>
+              && std::is_same_v<U, int>
+= {
+    return a * b;
+}
+
+main: () = {
+    x: X<int,int> = ();
+    std::cout << f<int,int>(2,5);
+}

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

@@ -0,0 +1 @@
+10

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

@@ -0,0 +1 @@
+abc

regression-tests/test-results/gcc-10/pure2-function-multiple-forward-arguments.cpp.execution

@@ -0,0 +1 @@
+10

regression-tests/test-results/gcc-10/pure2-function-multiple-forward-arguments.cpp.output

@@ -36,14 +36,14 @@ auto use(auto const& x) -> void{}
 
 // invoking each of these with an rvalue std::pair argument ...
 auto apply_implicit_forward(auto&& t) -> void
-requires std::is_same_v<CPP2_TYPEOF(t), std::pair<X,X>>
+requires (std::is_same_v<CPP2_TYPEOF(t), std::pair<X,X>>)
 #line 16 "mixed-forwarding.cpp2"
                                                      {
     copy_from(t.first);             // copies
     copy_from(CPP2_FORWARD(t).second);// moves
 }
 auto apply_explicit_forward(auto&& t) -> void
-requires std::is_same_v<CPP2_TYPEOF(t), std::pair<X,X>>
+requires (std::is_same_v<CPP2_TYPEOF(t), std::pair<X,X>>)
 #line 20 "mixed-forwarding.cpp2"
                                                      {
     copy_from(        CPP2_FORWARD(t).first);// moves

regression-tests/test-results/gcc-10/pure2-requires-clauses.cpp.execution

@@ -34,7 +34,7 @@ auto parameter_styles(
     std::string&& d, 
     auto&&   e
     ) -> void
-requires std::is_same_v<CPP2_TYPEOF(e), std::string>
+requires (std::is_same_v<CPP2_TYPEOF(e), std::string>)
 #line 15 "mixed-parameter-passing-with-forward.cpp2"
 {
     int z {12}; 

regression-tests/test-results/gcc-10/pure2-requires-clauses.cpp.output

@@ -0,0 +1 @@
+abc