Initial implementation of tuple demonstration

This allows allocating a buffer of double pointers pointing to
the memory location in a bunch of variables. It is not particularly
ergonomic as is, and it seems unlikely the real world performance
benefit will exist.

kiwi.lua

@@ -64,8 +64,16 @@ typedef struct KiwiErr {
    bool must_free;
 } KiwiErr;
 
+typedef struct KiwiTuple {
+   int count;
+   const double* values[?];
+} KiwiTuple;
+
 struct KiwiSolver;
 
+void kiwi_tuple_init(KiwiTuple* tuple, int count, ...);
+void kiwi_tuple_destroy(KiwiTuple* tuple);
+
 KiwiVar* kiwi_var_construct(const char* name);
 void kiwi_var_release(KiwiVar* var);
 void kiwi_var_retain(KiwiVar* var);
@@ -196,6 +204,21 @@ function kiwi.is_constraint(o)
    return ffi_istype(Constraint, o)
 end
 
+local Tuple = ffi.typeof("struct KiwiTuple") --[[@as kiwi.Tuple]]
+kiwi.Tuple = Tuple
+
+---@class kiwi.Tuple: ffi.cdata*
+---@field values ffi.cdata*
+---@field count integer
+---@overload fun(vars: kiwi.Var[]): kiwi.Tuple
+ffi.metatype(Tuple, {
+   __new = function(self, vars)
+      local t = ffi_new(self, #vars)
+      ljkiwi.kiwi_tuple_init(t, #vars, unpack(vars))
+      return ffi_gc(t, ljkiwi.kiwi_tuple_destroy)
+   end,
+})
+
 ---@param expr kiwi.Expression
 ---@param var kiwi.Var
 ---@param coeff number?