xs.js:
  - implement XS.add_exports() to add exports into XS
  - deprecate XS.export_code() replaced by XS.add_exports()
  - use XS.add_exports() instead of deprecated XS.export_code()
test/http.js: require_resolve expect.js instead of expect.js/expect.js after expect.js upgrade
file.js:
  - require_resolve(): fix trace
  - use XS.add_exports() instead of deprecated XS.export_code()