[naga] Break `naga::arena` up into submodules.

[jimblandy]

This is broken up into independent commits that are probably easier to review individually.

• [naga] Use Range::from_index_range in Arena::range_from.

  Use the associated function Range::from_index_range to construct the return value of Arena::range_from, rather than constructing the Range directly, in preparation for those fields becoming private.

• [naga] Use Handle::new, don't construct the Handle directly.

  In preparation for making Handle fields private to another module, have the Iterator implementation for arena::Range call Handle::new to construct the handle being produced, rather than building it using a struct literal.

• [naga] Move naga/src/arena.rs to naga/src/arena/mod.rs.

• [naga] Break naga::arena up into submodules.

  This commit is almost entirely code motion. The only meaningful changes should be:

  • changes to imports
  • changes to visibility
  • changes to use visible associated constructor functions instead of trying to construct values directly using now-invisible fields
  • moving the crate-level "Arena" docs into the arena module

[teoxoy]

It looks like there is a conflict since https://github.com/gfx-rs/wgpu/pull/5864 added #[derive(Debug)] to HandleVec.