add reusable boolean constants

This will save us having to reconstruct these repeatedly when we just want to
ask the question "is the expression I have the literal true or false?"

Github: related to #130 "interact with SMT solver"
Github: related to #139 "avoid calling SMT solver for "true" and "false""

librumur/include/rumur/Boolean.h

@@ -2,11 +2,15 @@
 
 #include <cstddef>
 #include <memory>
+#include <rumur/Expr.h>
 #include <rumur/Ptr.h>
 #include <rumur/TypeExpr.h>
 
 namespace rumur {
 
 extern const Ptr<Enum> Boolean;
 
+extern const Ptr<Expr> False;
+extern const Ptr<Expr> True;
+
 }

librumur/src/Boolean.cc

@@ -2,7 +2,9 @@
 #include "location.hh"
 #include <memory>
 #include <rumur/Boolean.h>
+#include <rumur/Decl.h>
 #include <rumur/Expr.h>
+#include <rumur/Number.h>
 #include <rumur/Ptr.h>
 #include <rumur/TypeExpr.h>
 #include <string>
@@ -15,4 +17,16 @@ const Ptr<Enum> Boolean = Ptr<Enum>::make(
   std::vector<std::pair<std::string, location>>(
     { {"false", location()}, {"true", location()} }), location());
 
+const Ptr<Expr> False = Ptr<ExprID>::make("false",
+  Ptr<ConstDecl>::make("boolean",
+    Ptr<Number>::make(0, location()),
+  Boolean, location()),
+location());
+
+const Ptr<Expr> True = Ptr<ExprID>::make("true",
+  Ptr<ConstDecl>::make("boolean",
+    Ptr<Number>::make(1, location()),
+  Boolean, location()),
+location());
+
 }