make function types as constraints usable

See #211.

spec/lang/declaration/local_function_spec.lua

@@ -1,4 +1,3 @@
-local tl = require("tl")
 local util = require("spec.util")
 
 describe("local function", function()
@@ -21,6 +20,16 @@ describe("local function", function()
          local ok = f(3, "abc")
       ]]))
 
+      it("can have type variables with constraints", util.check([[
+         local type F = function(any...): any...
+
+         local function partial<T is F>(f: T, a: any): T
+            return function(...: any): any...
+               return f(a, ...)
+            end
+         end
+      ]]))
+
       it("cannot have unused type variables", util.check_type_error([[
          local function f<Z>(a: number, b: string): ()
             return

tl.lua

@@ -8408,8 +8408,11 @@ do
 
    function TypeChecker:to_structural(t)
       assert(not (t.typename == "tuple"))
+      if t.typename == "typevar" and t.constraint then
+         t = t.constraint
+      end
       if t.typename == "nominal" then
-         return self:resolve_nominal(t)
+         t = self:resolve_nominal(t)
       end
       return t
    end

tl.tl

@@ -8408,8 +8408,11 @@ do
 
    function TypeChecker:to_structural(t: Type): Type
       assert(not t is TupleType)
+      if t is TypeVarType and t.constraint then
+         t = t.constraint
+      end
       if t is NominalType then
-         return self:resolve_nominal(t)
+         t = self:resolve_nominal(t)
       end
       return t
    end