Make the necessary changes to support concurrency in Miri.
This pull request makes the necessary changes to the Rust compiler to allow Miri to support concurrency:
1. Move stack from the interpretation context (`InterpCx`) to machine, so that the machine can switch the stacks when it changes the thread being executed.
2. Add the callbacks that allow the machine to generate fresh allocation ids for each thread local allocation and to translate them back to original allocations when needed. This allows the machine to ensure the property that allocation ids are unique, which allows using a simpler representation of the memory.
r? @oli-obk
cc @RalfJung
c6b55eed63