Remove naga's span and validate features

[teoxoy]

It's unclear why the spans are gated behind the span feature. They are essential for diagnostics, take almost no space, and all users of naga that I could find enable it or have it enabled because they depend on wgpu-core transitively.

The validate feature might have more value than span as it gates quite a bit more code than span but still gets enabled by wgpu-core unconditionally.

The main driving factor behind removing them is maintenance burden (having to always consider the case where those are not enabled).

Related (original PRs):

• Proof of concept for "span" feature, with WGSL & GLSL parsing augmented. naga#1092
• Compile the validation out of an executable naga#1437

[cwfitzgerald]

To add more context, I have personally been bitten by the lack of both these features silently causing issues without real connection to why the problem is happening.