Refactor DescriptorRequirements

[Rua]

Changelog:

### Breaking changes
Changes to `DescriptorRequirements`:
- The struct has been split into two levels: the per-binding `DescriptorBindingRequirements`, and the per-descriptor-index `DescriptorRequirements`.
- The `descriptor_requirements` method of various types has been renamed to `descriptor_binding_requirements` to match.
- `DescriptorBindingRequirements` has a single `descriptors` member instead of multiple `HashSet` members. This member is a `HashMap` with `Option<u32>` as the key type. The key `None` holds requirements for non-constant indices, while requirements for constant indices are stored as `Some`.
- Validation checks against the requirements, and pipeline barriers, now always include the requirements of the `None` key as well. This may result in false positives, but prevents false negatives.
- `DescriptorRequirements` now has `memory_read` and `memory_write` members, which hold a `ShaderStages` value for the stages which read or write the resource. This is used for more fine-grained pipeline barriers.

Not much to say here, it's pretty much in the changelog already.

[AustinJ235]

Looks good