rust: add pin-init API core

This API is used to facilitate safe pinned initialization of structs. It
replaces cumbersome `unsafe` manual initialization with elegant safe macro
invocations.

Due to the size of this change it has been split into six commits:
1. This commit introducing the basic public interface: traits and
   functions to represent and create initializers.
2. Adds the `#[pin_data]`, `pin_init!`, `try_pin_init!`, `init!` and
   `try_init!` macros along with their internal types.
3. Adds the `InPlaceInit` trait that allows using an initializer to create
   an object inside of a `Box<T>` and other smart pointers.
4. Adds the `PinnedDrop` trait and adds macro support for it in
   the `#[pin_data]` macro.
5. Adds the `stack_pin_init!` macro allowing to pin-initialize a struct on
   the stack.
6. Adds the `Zeroable` trait and `init::zeroed` function to initialize
   types that have `0x00` in all bytes as a valid bit pattern.

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In this section the problem that the new pin-init API solves is out...