use SLOT_DATA for response context instead of __store

Replace js_getthis() with js_getcurrentfunc() and SLOT_DATA for accessing
context in response methods. This allows methods like c.res.body to work
when passed directly to .then() without requiring .bind().

• Add js_heavy_mkfun() to create functions with attached data
• Add ANT_PTR() macro for pointer-to-jsval conversion
• Update res_body, res_header, res_status, res_html, res_json, res_notFound, res_redirect to use js_getcurrentfunc() + SLOT_DATA
• Update req_header, js_set_prop, js_get_prop similarly
• Remove response_ctx and headers properties from objects