Use fully-qualified name for size_of

[zhassan-aws]

Add core::mem:: prefix to all uses of size_of.

Background: in some cases, I get errors of the form:

   Compiling kani_core v0.58.0 (/~https://github.com/model-checking/kani#35015dce)
error[E0425]: cannot find function `size_of` in this scope
  --> /home/ubuntu/.cargo/git/checkouts/kani-0ce0dacf5e98886d/35015dc/library/kani/src/lib.rs:54:1
   |
54 | kani_core::kani_lib!(kani);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
   |
   = help: consider importing one of these items:
           core::mem::size_of
           crate::core_path::intrinsics::size_of
           crate::core_path::mem::size_of
           std::mem::size_of
   = note: this error originates in the macro `kani_core::ptr_generator` which comes from the expansion of the macro `kani_core::kani_lib` (in Nightly builds, run with -Z macro-backtrace for more info)

when adding the Kani library as a dependency. Adding core::mem:: fixes those issues.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[zhassan-aws]

Here are steps to reproduce this issue:

1. cargo new --edition 2021 kani-dep
2. cd kani-dep
3. cargo add kani_core kani --git /~https://github.com/model-checking/kani
4. cargo +nightly-2024-05-20 build
   This produces:

   Compiling proc-macro2 v1.0.93
   Compiling unicode-ident v1.0.14
   Compiling kani_macros v0.58.0 (/~https://github.com/model-checking/kani#35015dce)
   Compiling kani v0.58.0 (/~https://github.com/model-checking/kani#35015dce)
   Compiling quote v1.0.38
   Compiling syn v2.0.96
   Compiling proc-macro-error-attr2 v2.0.0
   Compiling proc-macro-error2 v2.0.1
   Compiling kani_core v0.58.0 (/~https://github.com/model-checking/kani#35015dce)
error[E0425]: cannot find function `size_of` in this scope
  --> /home/ubuntu/.cargo/git/checkouts/kani-0ce0dacf5e98886d/35015dc/library/kani/src/lib.rs:54:1
   |
54 | kani_core::kani_lib!(kani);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
   |
   = help: consider importing one of these items:
           core::mem::size_of
           crate::core_path::intrinsics::size_of
           crate::core_path::mem::size_of
           std::mem::size_of
   = note: this error originates in the macro `kani_core::ptr_generator` which comes from the expansion of the macro `kani_core::kani_lib` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0425]: cannot find function `size_of` in this scope
  --> /home/ubuntu/.cargo/git/checkouts/kani-0ce0dacf5e98886d/35015dc/library/kani/src/lib.rs:54:1
   |
54 | kani_core::kani_lib!(kani);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
   |
   = help: consider importing one of these items:
           core::mem::size_of
           crate::core_path::intrinsics::size_of
           crate::core_path::mem::size_of
           std::mem::size_of
   = note: this error originates in the macro `kani_core::ptr_generator_fn` which comes from the expansion of the macro `kani_core::kani_lib` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0425]: cannot find function `size_of` in this scope
  --> /home/ubuntu/.cargo/git/checkouts/kani-0ce0dacf5e98886d/35015dc/library/kani/src/lib.rs:54:1
   |
54 | kani_core::kani_lib!(kani);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
   |
   = help: consider importing one of these items:
           core::mem::size_of
           crate::core_path::intrinsics::size_of
           crate::core_path::mem::size_of
           std::mem::size_of
   = note: this error originates in the macro `kani_core::generate_models` which comes from the expansion of the macro `kani_core::kani_lib` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0658]: use of unstable library feature 'strict_provenance'
  --> /home/ubuntu/.cargo/git/checkouts/kani-0ce0dacf5e98886d/35015dc/library/kani/src/lib.rs:54:1
   |
54 | kani_core::kani_lib!(kani);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #95228 </~https://github.com/rust-lang/rust/issues/95228> for more information
   = help: add `#![feature(strict_provenance)]` to the crate attributes to enable
   = note: this compiler was built on 2024-05-19; consider upgrading it if it is out of date
   = note: this error originates in the macro `kani_core::generate_models` which comes from the expansion of the macro `kani_core::kani_lib` (in Nightly builds, run with -Z macro-backtrace for more info)

Some errors have detailed explanations: E0425, E0658.
For more information about an error, try `rustc --explain E0425`.
error: could not compile `kani` (lib) due to 8 previous errors

[celinval]

Can you add a test for this?

[zhassan-aws]

I added a test, although it doesn't exhibit the issue without the changes in this PR. The issues only appear with earlier toolchains (e.g. nightly-2024-05-20). The test I added ensures that adding the Kani library as a dependency works, so it's useful regardless.