Use usize::div_ceil instead of open-coding it

[yotamofek]

A small cleanup opportunity I found while working on something else.

BTW

the same file also has these two lines, twice:

let rem = self.v.len() % self.chunk_size;
let end = if rem == 0 { self.chunk_size } else { rem };

They can similarly be "simplified" to:

let end = self.v.len().min(self.chunk_size);

but I'm not convinced the majority of people will consider the one-liner is easier to read, so would love to hear opinions about that.

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