ARROW-8597 [Rust] Lints and readability improvements for arrow crate

[durch]

• Pedantic fixes to unsafe
• Changes to function arguments to pass in references or values as appropriate
• Refactor pointer arithmetic to use usize instead of isize casting
• Ignore generated code clippy warnings
• Refactor loops to use iterators where appropriate
• Remove unnecessary return statements
• Remove unnecessary clone statements
• Remove unnecessary closures
• A bunch of similar small scale refactorings

Tests below are currently failing on master locally as well with ARROW_TEST_DATA not defined: NotPresent, assuming that is ok for now:

ipc::reader::tests::read_generated_files
ipc::reader::tests::read_generated_streams
ipc::writer::tests::read_and_rewrite_generated_files
ipc::writer::tests::read_and_rewrite_generated_streams

[github-actions]

https://issues.apache.org/jira/browse/ARROW-8597

[paddyhoran]

LGTM, thanks @durch

One minor nit.

Looks like the lint check is failing CI. Once these are fixed we can get this merged.

[nevi-me]

Thanks @durch, I didn't see a clippy ignore for len() without is_empty(), or did you ignore that lint. I had recently come across it, think that is_empty() wouldn't be useful as we rarely have empty arrays.

[durch]

@nevi-me I left it as is for now, did not want to actually write any new code in this PR. After this one gets merged I'll make another pass and write some code, in addition to the is_empty there is also a low hanging Default impl, possibly a few more pedantic changes :).

[markhildreth]

@durch Re: "ARROW_TEST_DATA not defined", the /rust/README.md file in the repo has some additional steps needed to get access to those test files locally. Basically...

git submodule update --init

...then run cargo test with these environment variables set:

PARQUET_TEST_DATA=/path/to/arrow/cpp/submodules/parquet-testing/data
ARROW_TEST_DATA=/path/to/arrow/testing/data/