Tests do not pass miri due to alignment issues

[saethlin]

Describe the bug
The parquet bit_packing module does misaligned reads through a raw pointer, and murmur_hash2_64a constructs a misaligned reference. It's unclear to me if these qualify as security problems. They're definitely UB, and the fact that the UB is hit in tests suggests it's quite easy to hit in real code, but I can't imagine how one could construct an exploit with these.

running 470 tests
test arrow::array_reader::tests::test_complex_array_reader_def_and_rep_levels ... error: Undefined Behavior: accessing memory with alignment 1, but alignment 4 is required
    --> parquet/src/util/bit_packing.rs:150:12
     |
150  |     *out = (*in_buf) % (1u32 << 2);
     |            ^^^^^^^^^ accessing memory with alignment 1, but alignment 4 is required
     |
     = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
     = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

     = note: inside `util::bit_packing::unpack2_32` at parquet/src/util/bit_packing.rs:150:12

After patching up that:

test column::writer::tests::test_byte_array_statistics ... error: Undefined Behavior: accessing memory with alignment 2, but alignment 8 is required
    --> /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/raw.rs:95:14
     |
95   |     unsafe { &*ptr::slice_from_raw_parts(data, len) }
     |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ accessing memory with alignment 2, but alignment 8 is required
     |
     = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
     = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

     = note: inside `std::slice::from_raw_parts::<u64>` at /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/raw.rs:95:14
note: inside `util::hash_util::murmur_hash2_64a` at parquet/src/util/hash_util.rs:56:25
    --> parquet/src/util/hash_util.rs:56:25
     |
56   |       let data_bytes_64 = std::slice::from_raw_parts(
     |  _________________________^
57   | |         &data_bytes[0..len_64] as *const [u8] as *const u64,
58   | |         len / 8,
59   | |     );
     | |_____^
note: inside `util::hash_util::hash_` at parquet/src/util/hash_util.rs:32:13

For what it's worth, this comment:

/// SAFTETY Only safe on platforms which support unaligned loads (like x86_64)

is inaccurate. The ptr type documents this:

Working with raw pointers in Rust is uncommon, typically limited to a few patterns. Raw pointers can be unaligned or null. However, when a raw pointer is dereferenced (using the * operator), it must be non-null and aligned.

Misaligned access through a pointer is UB on all platforms.

To Reproduce
MIRIFLAGS="-Zmiri-disable-isolation" cargo +nightly miri test (this requires a fair bit of patience, like most projects this crate's tests were not designed with a super-slow interpreter in mind).

Expected behavior
Miri does not detect any UB.

Additional context
It seems like the first of these has been known for some time, but actually fixing it was punted on in e98efae.

I suspect there is other UB. The code surrounding these cases follows similar patterns, but I'm not yet sure if miri doesn't detect anything else because the code is fine or because the bad cases aren't covered by tests.