Better validation and errors

[Rua]

After #1964 was merged I realised that there are still some shortcomings. This PR fixes those:

• Ability to specify instance extensions, or API version only, for the bitflags and enum macros.
• Ability to validate bitflags and enums with a physical device or an instance too.
• Standardised error types across Vulkano for reporting that something is required from the instance/device but not available (RequirementNotMet), and standardised the wording of the messages.
• Use a new RequiresOneOf struct type that can specify multiple possible requirements that may satisfy a requirement. This is useful for when extensions get promoted, and so there are multiple ways to get functionality.
• Some random formatting/style changes.

This is all internal stuff, other than the error types, but changes to errors haven't been listed in the changelog for a while now anyway.

[AustinJ235]

Really like the RequiresOneOf type, great idea!