[Rust] Soundness issues via `Buffer` methods

[asfimport]

This is my first time creating an issue, so please let me know if I need to do anything differently.

There are a few soundness issues with the methods currently available on Buffer.

1. Using a combination of from_raw_parts and data/as_ref, e.g. Buffer::from_raw_parts(ptr, len).data(), it's possible to dereference arbitrary memory locations, break pointer aliasing rules, etc. To fix this, from_raw_parts needs to be unsafe, and the safety requirements on ptr and len should be specified. (For an example of a similar method in the standard library, see std::slice::from_raw_parts.)
2. By implementing the ArrowNativeType trait on a struct, it's possible for a user to create invalid values of that struct using the typed_data method. To fix this, the ArrowNativeType trait needs to be unsafe, or users need to be prevented from implementing ArrowNativeType on arbitrary types. Alternatively, the typed_data method could be made unsafe.
3. It's possible to create invalid values of the bool type using typed_data. (Values of bool must be 0x00 or 0x01; arbitrary u8 cannot safely be reinterpreted as bool.) To fix this, typed_data::<bool>() needs to iterate over all the data and check that all the elements are valid, or typed_data needs to be marked unsafe.

Reporter: Jim Turner
Assignee: Paddy Horan / @paddyhoran

PRs and other links:

• GitHub Pull Request #6397

_{Note: This issue was originally created as ARROW-7624. Please see the migration documentation for further details.}

[asfimport]

Paddy Horan / @paddyhoran:
Thanks for reporting this [~jturner314].

We recently had a discussion about these kinds of issues on the [mailing list|[https://lists.apache.org/thread.html/r07f4ba469563a764d19fd08622adbcd0e3ac895a6e8165ae44b8dee8%40%3Cdev.arrow.apache.org%3E].]  I'm going to start trying to clean them up soon.

[asfimport]

Paddy Horan / @paddyhoran:
Issue resolved by pull request 6397
#6397