Alignment of the bytes of Allocation to match align parameter

[emarteca]

Aligning the bytes field of Allocation so that it is aligned matching the align parameter.

Main changes are to:

• in the Allocation constructors from_bytes and uninit
• in the adjust_from_tcx method in Allocation

Instead of just converting the relevant [u8] to a Box<[u8]> directly we now:

• create a std::alloc::Layout of the alignment the Allocation is specified to have
• allocate a buffer with this layout
• get the corresponding [u8] for this buffer

Then we write the relevant bytes to it, check for uninit, etc -- same process as previously.

NOTE
With this current code, when the bytes field is deallocated, even though the size is right, in the layout the alignment might be under-required: for example we might have allocated it with alignment 4 and deallocate it with alignment 2. We know this is undefined behaviour violating the memory fitting requirements of dealloc -- we're going to work around this by adding a wrapper struct for the Box<[u8]> that stores the alignment it was allocated with, and manually implements deallocation with the right alignment.

Why are we doing this?

We want the address of the bytes field of an Allocation to be accessible, and represent the actual address of the [u8]. This is part of an effort to pass pointers in Miri to the C FFI.