Give FileDescription::{read, write} access to the InterpCx

[RalfJung]

The read and write methods of FileDescriptor should have access to ecx: &mut InterpCx<'mir, 'tcx, MiriMachine<'mir, 'tcx>>. This will be needed in the future to support blocking on these operations.

This is a prerequisite for

• Complete socketpair support #3442
• Complete and fix our eventfd implementation #3445

To make that work out in terms of borrowing, I think what we will have to do is clone the Rc in the read/write functions in unix/fd.rs, so that we have a fully owned FileDescriptor next to a uniquely borrowed InterpCx.

Cc @tiif @Luv-Ray
(AFAIK @tiif is still in the design stage so this is not a blocker yet)

[RalfJung]

Thinking about it, supporting blocking will need further changes -- it's more tricky than I thought. After all, we need to arrange for the waking thread to be able to fill the buffer and set the return value before waking up the blocked thread.

But giving mutable access to the interpreter is probably still a good first step.