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)

correctly normalize constants

closes rust-lang#70317

implements rust-lang#70125 (comment)

r? eddyb cc @varkor

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

avoid catching InterpError

Avoid raising and then capturing `InterpError` for the definedness check.

Cc rust-lang#69297
r? @oli-obk

fix incorrect type name in doc comments

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