Handling pointer provenance in Pulley

[alexcrichton]

This started out as https://github.com/bytecodealliance/wasmtime/issues/9015 but I'm evolving this now to be about Pulley specifically. Current thinking is that Pulley will not be compatible with -Zmiri-strict-provenance, but changes are necessary to make it compatible with -Zmiri-permissive-provenance. I believe the concrete of changes are:

• The ptr: *mut u8 field of XRegValUnion must be replaced with a usize value
• The new_ptr constructor will use expose_provenance and load/store usize
• The get_ptr accessor will use with_exposed_provenance with the usize field
• When entering the interpreter Wasmtime will be require to call expose_provenance on any pointers that Pulley may access. This notably affects the VMContext field and any transitive pointers that it contains. This can probably be done during the construction of VMContext itself.

Right now expose_provenance and with_exposed_provenance are unstable so I don't believe that this is actionable at this time, but I wanted to write this down for later.

cc @fitzgen

[alexcrichton]

Ralf also pointed to https://github.com/rust-lang/unsafe-code-guidelines/issues/497 which has some interesting discussion and I believe reaches a similar conclusion.

[fitzgen]

Ralf also pointed to rust-lang/unsafe-code-guidelines#497 which has some interesting discussion and I believe reaches a similar conclusion.

This comment from Ralf, in particular, seems to summarize our recent discussions: https://github.com/rust-lang/unsafe-code-guidelines/issues/497#issuecomment-2003100550