* Most noticeably, shortened std::ffi::CStr[ing] and ::std::os::raw::c_void.
* (qmetaobject/src/scenegraph.rs):
  - Removed probably misplaced #[cfg(qt_5.8)] attribute before use super::*;
  - Fixed indentation in screnegraph, because cargo fmt could be wrong when
    it comes to  macros.

Replaces all `extern crate ...` and especially #[macro_use] statements
with direct `use`s of relevant items.

Introduces cstr! crate & macro to examples, because we don't want our dear
users/readers to suffer with raw CStr nightmare:
    std::ffi::CStr::from_bytes_with_nul(b"...\0").unwrap()