Fix clippy warnings for Rust 1.79

[wks]

This PR fixes new warnings generated by Clippy after upgrading Rust to 1.79.

• Only define ImmixSpaceArg::mixed_age when the "vo_bit" feature is enabled. This eliminates a dead code warning.
• Use the associated constant <integer_type>::MAX (and MIN, too) instead of the deprecated max_value() method and the ::std::<integer_type>::MAX constant.
• Annotate the destination type of some transmute calls.
• Use the safe std::ptr::NonNull::from() converter instead of transmute.
• Initialize array elements of type Atomic<MapState> from constant.
• Remove MutatorContext::barrier_impl. It is not used by any bindings right now, and it depends on the layout of &dyn Trait. Since Barrier has Downcast as a supertrait, if a binding needs a reference to a concrete Barrier implementation, it should just use downcast.

The warning against the transmute call in mock_vm.rs is suppressed because it is almost impossible to annotate.

Also fixes compilation problems:

• Made benchmarks related to the "mock_test" feature conditionally compiled so that we don't get compilation error when running cargo clippy --benches or cargo clippy --all-targets.

[wks]

Public API Check failed because I removed MutatorContext::barrier_impl(). DEFAULT_STRESS_FACTOR is not changed, but cargo-public-api thought its type was _.