Minor fixes and API change

- fix stupid spaces in Makefile
- small type annotation fix
- Modify Expression constructor API to use varargs
- Update kiwi.lua to support future C API module

kiwi.lua

@@ -1,6 +1,15 @@
-local kiwi = {}
+-- kiwi.lua - LuaJIT FFI bindings with C API fallback to kiwi constraint solver.
 
-local ffi = require("ffi")
+local ffi
+do
+   local ffi_loader = package.preload["ffi"]
+   if ffi_loader == nil then
+      return require("ckiwi")
+   end
+   ffi = ffi_loader() --[[@as ffilib]]
+end
+
+local kiwi = {}
 
 local ckiwi
 do
@@ -101,7 +110,7 @@ local ffi_copy, ffi_gc, ffi_istype, ffi_new, ffi_string =
 local concat = table.concat
 local has_table_new, new_tab = pcall(require, "table.new")
 if not has_table_new or type(new_tab) ~= "function" then
-   new_tab = function()
+   new_tab = function(_, _)
       return {}
    end
 end
@@ -532,7 +541,7 @@ end
 do
    --- Expressions are a sum of terms with an added constant.
    ---@class kiwi.Expression: ffi.cdata*
-   ---@overload fun(terms: kiwi.Term[], constant: number?): kiwi.Expression
+   ---@overload fun(constant: number, ...: kiwi.Term): kiwi.Expression
    ---@field constant number
    ---@field package term_count number
    ---@field package terms_ ffi.cdata*
@@ -610,7 +619,8 @@ do
    function Expression_cls:value()
       local sum = self.constant
       for i = 0, self.term_count - 1 do
-         sum = sum + self.terms_[i]:value()
+         local t = self.terms_[i]
+         sum = sum + t.var:value() * t.coefficient
       end
       return sum
    end
@@ -636,17 +646,16 @@ do
       __index = Expression_cls,
    }
 
-   function Expression_mt.__new(T, terms, constant)
-      local term_count = terms and #terms or 0
+   function Expression_mt.__new(T, constant, ...)
+      local term_count = select("#", ...)
       local e = ffi_gc(ffi_new(T, term_count), ckiwi.kiwi_expression_del_vars) --[[@as kiwi.Expression]]
       e.term_count = term_count
-      e.constant = constant or 0.0
-      if terms then
-         for i, t in ipairs(terms) do
-            local dt = e.terms_[i - 1] --[[@as kiwi.Term]]
-            dt.var = ckiwi.kiwi_var_clone(t.var)
-            dt.coefficient = t.coefficient
-         end
+      e.constant = constant
+      for i = 1, term_count do
+         local t = select(i, ...)
+         local dt = e.terms_[i - 1] --[[@as kiwi.Term]]
+         dt.var = ckiwi.kiwi_var_clone(t.var)
+         dt.coefficient = t.coefficient
       end
       return e
    end
@@ -729,7 +738,7 @@ do
    ---@return kiwi.Expression
    ---@nodiscard
    function Constraint_cls:expression()
-      local SZ = 8
+      local SZ = 7 -- 2**7 bytes on x64
       local expr = ffi_new(Expression, SZ) --[[@as kiwi.Expression]]
       local n = ckiwi.kiwi_constraint_expression(self, expr, SZ)
       if n > SZ then
@@ -865,7 +874,7 @@ do
    --- Produce a custom error raise mask
    --- Error kinds specified in the mask will not cause a lua
    --- error to be raised.
-   ---@param kinds (kiwi.ErrKind|number)[]
+   ---@param kinds (kiwi.ErrKind|integer)[]
    ---@param invert boolean?
    ---@return integer
    function kiwi.error_mask(kinds, invert)
@@ -948,7 +957,7 @@ do
    end
    ---@class kiwi.Solver: ffi.cdata*
    ---@field package error_mask_ integer
-   ---@overload fun(error_mask: (integer|(kiwi.ErrKind|number)[] )?): kiwi.Solver
+   ---@overload fun(error_mask: (integer|(kiwi.ErrKind|integer)[] )?): kiwi.Solver
    local Solver_cls = {
       --- Test whether a constraint is in the solver.
       ---@type fun(self: kiwi.Solver, constraint: kiwi.Constraint): boolean
@@ -978,7 +987,7 @@ do
    }
 
    --- Sets the error mask for the solver.
-   ---@param mask integer|(kiwi.ErrKind|number)[] the mask value or an array of kinds
+   ---@param mask integer|(kiwi.ErrKind|integer)[] the mask value or an array of kinds
    ---@param invert boolean? whether to invert the mask if an array was passed for mask
    function Solver_cls:set_error_mask(mask, invert)
       if type(mask) == "table" then