The former `done` flag was roughly similar to an `Option` tag, but left
the possibity of misuse. By using a real `Option`, we can set `None`
when the iterator is exhausted, removing any way to call it again. We
also allow niche layout this way, so the `Fuse` may be smaller.

The `FusedIterator` specialization does want to ignore the possibility
of exhaustion though, so it uses `unsafe { intrinsics::unreachable() }`
to optimize that branch away. The entire `Fuse` implementation is now
isolated in its own module to contain that unsafety.

InterpCtx => InterpCx 
(rustc_mir::interpret::InterpCx)

…rand of overflowing_signed_offset

use checked casts and arithmetic in Miri engine

This is unfortunately pretty annoying because we have to cast back and forth between `u64` and `usize` more often that should be necessary, and that cast is considered fallible.

For example, should [this](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/mir/interpret/value/enum.ConstValue.html) really be `usize`?

Also, `LayoutDetails` uses `usize` for field indices, but in Miri we use `u64` to be able to also handle array indexing. Maybe methods like `mplace_field` should be suitably generalized to accept both `u64` and `usize`?

r? @oli-obk Cc @eddyb

correctly normalize constants

closes rust-lang#70317

implements rust-lang#70125 (comment)

r? eddyb cc @varkor

…-code-e0710, r=Dylan-DPC

Add long error explanation for E0710

Add long explanation for the E0710 error code
Part of rust-lang#61137

r? @GuillaumeGomez

Implement Fuse with Option

The former `done` flag was roughly similar to an `Option` tag, but left
the possibity of misuse. By using a real `Option`, we can set `None`
when the iterator is exhausted, removing any way to call it again. We
also allow niche layout this way, so the `Fuse` may be smaller.

The `FusedIterator` specialization does want to ignore the possibility
of exhaustion though, so it uses `unsafe { intrinsics::unreachable() }`
to optimize that branch away. The entire `Fuse` implementation is now
isolated in its own module to contain that unsafety.

r? @scottmcm

fix incorrect type name in doc comments

Change : `InterpCtx` => `InterpCx`
(`rustc_mir::interpret::InterpCx`)