#166 always goes the slow path for most types

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bytemuck's try_cast_vec for Vec<u8>->Vec<u16> or any different alignment will fail, regardless of whether the allocation is aligned. It is undefined behaviour in rust to deallocate with a different alignment than the allocation. So this code will always take the second path. That could be changed to

Ok(TypedArray::UInt16(
    try_cast_slice(data[..])
    .map(|s| s.to_vec())  // this copy already always happens
    .unwrap_or_else(
                    |(_, data)| {
                        // Fallback to manual conversion when not aligned
                        data.chunks_exact(2)
                            .map(|b| u16::from_ne_bytes([b[0], b[1]]))
                            .collect()
                    },
                )))

which will go the first path if the allocation is aligned.

Alternatively, this copy could be avoided with some notes in #203

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I made a quick PR (#235) that solves it quick-and-dirty, but
I think the ideal pattern would be:

let mut typed_array = TypedArray::new(dtype, size);
fill_with_important_data(typed_array.as_mut());
// done

that saves copies on non-aligned casts and is infallible.