All kinds of things

- Add modified BSD license
- Initial test runner github action
- Add is_{type} functions
- export kiwi.Error metatable such that it appears like a class.
- Allow Expression.new to take nil for terms
- Initial set of specs

ckiwi/ckiwi.cpp

@@ -248,8 +248,8 @@ void kiwi_var_set_value(KiwiVarRef var, double value) {
       self->setValue(value);
 }
 
-int kiwi_var_eq(KiwiVarRef var, KiwiVarRef other) {
-   VariableRef self(var);  // const defect in upstream
+bool kiwi_var_eq(KiwiVarRef var, KiwiVarRef other) {
+   ConstVariableRef self(var);  // const defect in upstream
    const VariableRef other_ref(other);
 
    return self && other_ref && self->equals(other_ref);