Miri Memory Work

Miri Memory Work

* Unify the two maps in memory to store the allocation and its kind together.
* Share the handling of statics between CTFE and miri: The miri engine always
  uses "lazy" `AllocType::Static` when encountering a static.  Acessing that
  static invokes CTFE (no matter the machine).  The machine only has any
  influence when writing to a static, which CTFE outright rejects (but miri
  makes a copy-on-write).
* Add an `AllocId` to by-ref consts so miri can use them as operands without
  making copies.
* Move responsibilities around for the `eval_fn_call` machine hook: The hook
  just has to find the MIR (or entirely take care of everything); pushing the
  new stack frame is taken care of by the miri engine.
* Expose the intrinsics and lang items implemented by CTFE so miri does not
  have to reimplement them.