Allows viewing and modifying arbitrary structs as bytes

Affected versions of rgb crate allow viewing and modifying data of any type T wrapped in RGB<T> as bytes,
and do not correctly constrain RGB<T> and other wrapper structures to the types for which it is safe to do so.

Safety violation possible for a type wrapped in RGB<T> and similar wrapper structures:

• If T contains padding, viewing it as bytes may lead to exposure of contents of uninitialized memory.
• If T contains a pointer, modifying it as bytes may lead to dereferencing of arbitrary pointers.
• Any safety and/or validity invariants for T may be violated.

The issue was resolved by requiring all types wrapped in structures provided by RGB crate to implement an unsafe marker trait.