Log Message:
cbindgen: Update to 0.15.0

Changelog:
  * Allow customizing mangling of generic parameters in C (#575)
  * Box<T> simplifies to T* in C (4ce324c)
  * ManuallyDrop<T> and MaubeUninit<T> simplify to T in C, and are \ 
opaque in C++ (0076a17)
  * C++ supports a derive-ostream annotation to derive serialization of structs, \ 
unions and enums (#582)
  * Large character constants have been fixed on Windows (#586)
  * Constants are now generated for typedefs, etc (#589)
  * The `sort_by` configuration option has been made to work for constants (#587)
  * Default sort order is source order now (sort_by = "None"), and can \ 
be changed by the above option (#587)