Add getters and From implementations to BufferSlice.

[kpreid]

Connections
Part of addressing #6974

Description
This PR adds functions to make BufferSlice usable in more situations.

• BufferSlice::buffer(), BufferSlice::offset() and BufferSlice::size() allow decomposing a BufferSlice. This allows a function which receives a BufferSlice to use it, for example, in texture copy operations. I'd like to have a more ergonomic interface to that, but I haven’t thought of one; adding these getters ensures that there is never any unavoidable inexpressiveness. However,
  • It is arguably undesirable to allow going from a BufferSlice to its Buffer and accessing data outside the slice. I think that this is worth considering, but that on net it is more important to be flexible than to be strict, here, particularly as buffers are interior mutable and clonable so there is no fundamental mutability-xor-sharing like &mut [u8] would have.
  • BufferSlice::size() returns a BufferSize rather than an Option<BufferSize>. See Refactor: eliminate unnecessary Option<BufferSize> #7147 for context on why I think this is the right choice — the Option should, I think, be considered an obsolete implementation detail.
• impl From<BufferSlice> for BufferBinding
• impl From<BufferSlice> for BindingResource

Please merge this PR without squashing; the commits are self-contained.

Testing
Added and ran API tests.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.