kaidokert / fixed-bigint-rs / 29183697479

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Fix panic-freeness of holder_be/holder_le under NumToBytes at -Oz (#136)

* holder_be/holder_le: inline chunks_exact_mut, drop the fallible route

`num_traits::ToBytes::to_be_bytes(&FixedUInt)` (and its LE sibling)
route through the `holder_be` / `holder_le` crate-private helpers.
Before this fix, those helpers called the fallible slice-based
`to_be_bytes(&mut [u8]) -> Result<&[u8], bool>` at `fixeduint.rs:556`,
whose runtime `if output_buffer.len() < total_bytes { return Err(false); }`
guard fails to DCE at `opt-level = z` + LTO on multi-byte limbs.
Every downstream that reaches `NumToBytes::to_be_bytes` on FixedUInt
inherits a `panic_fmt` reachable in the linked binary, even though
`BytesHolder::as_byte_slice_mut()` returns a slice of statically-known
length `N * size_of::<T>()`.

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