feat(bootstrap): function pointer types + C emission

- Add tekFunc AST node and parser support for func(Params) -> Ret
- Add tkFunc type resolution in sema and hir_lower
- Add &FuncName address-taking yielding tkMutRef(tkFunc)
- Fix C emission: function pointer params use Ret (*name)(Params) syntax
  via cParamDecl helper that embeds the name inside (*)
- Fix forward declarations and temp declarations for function pointers
- Add funcPtrTypes table in C backend so &Fn temps get proper type
- Add _test_funcptr integration test

_test_funcptr/bux.toml

@@ -0,0 +1,2 @@
+name = "test_funcptr"
+version = "0.1.0"

_test_funcptr/src/Main.bux

@@ -0,0 +1,17 @@
+module Main {
+
+func Apply(x: int, op: func(int) -> int) -> int {
+    return op(x);
+}
+
+func Double(n: int) -> int {
+    return n * 2;
+}
+
+func main() -> int {
+    let result: int = Apply(5, &Double);
+    if result != 10 { return 1; }
+    return 0;
+}
+
+}