WIP: add deletion files to Lance format

[wjones127]

Closes #908

[chebbyChefNEQ]

I think this looks good. I never reviewed rust before. I will do a second pass in the morning just to make sure I'm not missing something obvious.

[chebbyChefNEQ]

potential tangent: maybe we want 64 bit uint to be safe? Just so that we don't have a "small" limit of number of rows we can store.

[wjones127]

So this isn't well documented, but the row ids elsewhere are a concatenation of the fragment id and the within-fragment row id (which is the position / offset of the row in the file).

lance/rust/src/io/reader.rs

Lines 120 to 123 in 505dbcd

	/// Compute row id from `fragment_id` and the `offset` of the row in the fragment.
	fn compute_row_id(fragment_id: u64, offset: i32) -> u64 {
	(fragment_id << 32) + offset as u64
	}

So I think we already have a limit on the number of rows we can store per file based on this.

[chebbyChefNEQ]

makes sense. thank you for explaining!