Validation function for the IR

[kvark]

The Module struct that our front-ends produce generally has some level of type-level encoding of the constraints, however many properties can only be checked at run-time. One big area of validation is type checking:

• builtins have the expected types
• constructors are using consistent values, e.g. WGSL doesn't type check values for vectors #43
• types generally agree between producer and consumer

Another area is access checking:

• all the handles are actually valid
• input globals can only be read
• can't linearly sample integer textures
• depth images can only be used with comparison samplers
• and more!

Finally we need to enforce some of the conventions. For example, variables can either be stored as pointers (that are loaded/stored), or directly. There is currently no consistency, and whatever we choose would need to be enforced.

We have a proc module for end-point-independent processing. I think the validator sub-module could live there. The main task of it would be getting a module and making sure that it's valid. We'd be running it in CI tests to make sure that the module contents produced by the front-ends, or by different processors/transformers, are all valid.