this means we can get rid of the public allocate_op, and make OpTy only
constructible in librustc_mir

* 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.

This is still roughly 45ns slower than the old state, because it now works with
an MPlaceTy and uses the appropriate abstractions, instead of working with a
ptr-align pair directly.

- The logging in step.rs becomes debug! to make it stand out a bit more
- Dump value of operands (in `eval_operands`)
- Try to log a bit less verbose

* Value gets renamed to Operand, so that now interpret::{Place, Operand} are the
  "dynamic" versions of mir::{Place, Operand}.
* Operand and Place share the data for their "stuff is in memory"-base in a new
  type, MemPlace.  This also makes it possible to give some more precise types
  in other areas.  Both Operand and MemPlace have methods available to project
  into fields (and other kinds of projections) without causing further
  allocations.
* The type for "a Scalar or a ScalarPair" is called Value, and again used to
  give some more precise types.
* All of these have versions with an attached layout, so that we can more often
  drag the layout along instead of recomputing it.  This lets us get rid of
  `PlaceExtra::Downcast`.  MPlaceTy and PlaceTy can only be constructed
  in place.rs, making sure the layout is handled properly.
  (The same should eventually be done for ValTy and OpTy.)
* All the high-level functions to write typed memory take a Place, and live in
  place.rs.  All the high-level typed functions to read typed memory take an
  Operand, and live in operands.rs.

This reverts commit 59d21c526c036d7097d05edd6dffdad9c5b1cb62, and uses
tuple to store the mutable parts of an EvalContext (which now includes
`Machine`). This requires that `Machine` be `Clone`.

I use a pattern binding in each custom impl, so that adding fields to
`Memory` or `Frame` will cause a compiler error instead of causing e.g.
`PartialEq` to become invalid. This may be too cute.

This adds several requirements to `Machine::MemoryData`. These can be
removed if we don't want this associated type to be part of the equality
of `Memory`.

Change surrounding code to use accessor methods to refer to these fields.
Similar changes have not yet been made in tools/miri