[Rust] [Parquet] Fix compiler warning about unused return value

[asfimport]

When compiling with latest nightly, this warning is produced:

warning: unused return value of `std::mem::replace` that must be used
   --> parquet/src/encodings/encoding.rs:391:9
    |
391 |         mem::replace(&mut self.hash_slots, new_hash_slots);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_must_use)]` on by default
    = note: if you don't need the old value, you can just assign the new value directly 

Reporter: Andy Grove / @andygrove
Assignee: Neville Dipale / @nevi-me

_{Note: This issue was originally created as ARROW-10108. Please see the migration documentation for further details.}

[asfimport]

Neville Dipale / @nevi-me:
There are a lot of clippy issues with the latest nightly too, perhaps we could move to the latest nightly after the 2.0.0 release? I'd also like for us to perform destructive refactors soon as possible after the release (rearranging the arrow::array module by splitting arrays into their own files)

[asfimport]

Neville Dipale / @nevi-me:
This was fixed as part of the nightly update