feat(kernel/constraint): add 'print' function for debugging purposes

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/kernel/constraint.cpp

@@ -115,3 +115,4 @@ std::ostream & operator<<(std::ostream & out, constraint const & c) {
     return out;
 }
 }
+void print(lean::constraint const & c) { std::cout << c << "\n"; }