v0.2.5

• Fix docs.rs build for zng-view-prebuilt, zng-app, zng-wgt.
• Unlock cc dependency version.
• Remove crate features auto generated for optional dependencies.
• Add zng::app::print_tracing.
• In debug builds, prints info, warn and error tracing events if no tracing subscriber is set before the first call to APP.defaults or
  APP.minimal.