* 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.
* 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.
