Support container transmutation via macro

[joshlf]

This is one possible avenue for supporting #114.

A draft implementation is in #1736.

Most container transmutations require three components:

1. Src: FromBytes + IntoBytes + Immutable and Dst: FromBytes + IntoBytes + Immutable
2. size_of::<Src>() == size_of::<Dst>() and align_of::<Src>() == align_of::<Dst>()
3. Some way of invoking the right into_raw and from_raw associated functions on the container type

Currently, macros like transmute_ref! have the ability to enforce conditions (1) and (2). It should be possible in principle to support condition (3) using a trait:

#[doc(hidden)]
pub unsafe trait Container {
    type Element;
    type Raw;

    fn into_raw(self) -> Self::Raw;
    unsafe fn from_raw(Self::Raw) -> Self;
}

The associated Raw type is needed because not all containers use the same raw representations. For example, Arc uses *const T while Vec uses (*mut T, usize, usize).

A few open questions:

• In order to perform the size and alignment checks, we need Rust to be able to infer the types of certain variables. This is easy with transmute_ref!, but how would we do it when those types are inside containers? Off the top of my head, I think it should be possible via a function like fn unwrap<C: Container>(c: C) -> C::Element.
• Can we relax the trait bounds only in certain cases? E.g., Arc only requires Src: IntoBytes + Immutable and Dst: FromBytes + Immutable, but does not require Src: FromBytes or Dst: IntoBytes