ValidationError-ify commands, add some validation to UnsafeCommandBufferBuilder

[Rua]

Changelog:

### Breaking changes
Changes to command buffers:
- All validated binding and dynamic state commands now return `Result` instead of panicking, just like the other commands.
- The `begin_render_pass`, `next_subpass` and `end_render_pass` commands now take `SubpassBeginInfo` and/or `SubpassEndInfo` parameters. Nothing has changed functionally, but this allows for future-compatibility.
- `UnsafeCommandBufferBuilder` now performs some validation, when it does not rely on tracking any state. The old, completely unvalidated commands have been renamed with `_unchecked` added to the end.

Changes to `Format`:
- The `type_color`, `type_depth` and `type_stencil` methods are renamed to `numeric_format_color`, `numeric_format_depth` and `numeric_format_stencil`, and the returned type is renamed to `NumericFormat`.
- `ShaderScalarType` is renamed to `NumericType`.

[marc0246]

LGTM!