More flexible error handling, convenience methods

Allow getting solver errors returned rather than raising error.
The API allows setting a mask of which error kinds raise vs return.

Also add some convenience methods:
- `add_to` and `remove_from` constraint methods to solver.
- add and remove multiple constraints and edit/suggest variables at once

ckiwi/ckiwi.h

@@ -46,8 +46,8 @@ typedef struct KiwiErr {
    bool must_free;
 } KiwiErr;
 
-typedef struct KiwiSolver KiwiSolver;
-
+typedef struct KiwiSolver
+    KiwiSolver;  // LuaJIT: typedef struct { unsigned error_mask; } KiwiSolver;
 KiwiVarRef kiwi_var_new(const char* name);
 void kiwi_var_del(KiwiVarRef var);
 KiwiVarRef kiwi_var_clone(KiwiVarRef var);
@@ -74,8 +74,8 @@ enum KiwiRelOp kiwi_constraint_op(KiwiConstraintRef constraint);
 bool kiwi_constraint_violated(KiwiConstraintRef constraint);
 int kiwi_constraint_expression(KiwiConstraintRef constraint, KiwiExpression* out, int out_size);
 
-KiwiSolver* kiwi_solver_new();
-void kiwi_solver_del(KiwiSolver* sp);
+KiwiSolver* kiwi_solver_new(unsigned error_mask);
+void kiwi_solver_del(KiwiSolver* s);
 
 const KiwiErr* kiwi_solver_add_constraint(KiwiSolver* sp, KiwiConstraintRef constraint);
 const KiwiErr* kiwi_solver_remove_constraint(KiwiSolver* sp, KiwiConstraintRef constraint);