[naga] Clarify `Debug` form of `arena::Range`.

[jimblandy]

Change the std::fmt::Debug impl for naga::arena::Range to print a start-inclusive, end-exclusive range.

• This is more consistent with Rust's std::ops::Range.

• This is consistent with the serialization form used in snapshots, which simply uses the serialization of std::ops::Range.

• It is not consistent with Naga's constructor function Range::new_from_bounds, which takes an inclusive end value, or with Range::first_and_last, which returns an inclusive end value. Both of these need to represent ranges' end points as Handles, but exclusive end values might not be valid Handle values. I think this divergence is tolerable.

[teoxoy]

• It is not consistent with Naga's constructor function Range::new_from_bounds, which takes an inclusive end value, or with Range::first_and_last, which returns an inclusive end value. Both of these need to represent ranges' end points as Handles, but exclusive end values might not be valid Handle values. I think this divergence is tolerable.

I think the divergence makes sense in this case.

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This change looks consistent with the way we index in the Arena as well.

https://github.com/gfx-rs/wgpu/blob/a5d57db2694e73bda6dbfdf691d6b7fab267d554/naga/src/arena/mod.rs#L284