Align decimals (Int128) during IPC writes to enable zero-copy reads

[nameexhaustion]

Description

This currently isn't the case so we have to pay a copy when reading decimals.

Reproducible Example

The copy occurs here -

polars/crates/polars-arrow/src/mmap/array.rs

Lines 302 to 310 in c3c5bef

	} else {
	let mut values = vec![P::default(); num_rows];
	unsafe {
	std::ptr::copy_nonoverlapping(
	bytes.as_ptr(),
	values.as_mut_ptr() as *mut u8,
	bytes.len(),
	)
	};

. To observe the copy, set a breakpoint / add a panic on that line, and then write/read a decimal column to an IPC file:

import polars as pl
from decimal import Decimal

df = pl.Series("x", [Decimal("1.0")], dtype=pl.Decimal(18, 2)).to_frame()
df.write_ipc(path := ".env/data.ipc")
pl.read_ipc(path)

Although, I suspect this combination of decimal / IPC usage isn't very common, and even then you'd have to be throwing a very large amount of data to observe memory issues, so to me this is a low (or maybe even goal) priority.