Add extension information to instructions

[khyperia]

The spirv spec JSON has, for example, this information:

{
  "opname" : "OpDemoteToHelperInvocationEXT",
  "class"  : "Reserved",
  "opcode" : 5380,
  "capabilities" : [ "DemoteToHelperInvocationEXT" ],
  "extensions" : [ "SPV_EXT_demote_to_helper_invocation" ],
  "version" : "None"
},

{
  "enumerant" : "DemoteToHelperInvocationEXT",
  "value" : 5379,
  "capabilities" : [ "Shader" ],
  "extensions" : [ "SPV_EXT_demote_to_helper_invocation" ],
  "version" : "None"
},

This exposes the extensions json field in both inst.class.extensions, as well as, for example, capability.required_extensions() (both those two types of JSON extension information). This is useful for various things in rust-gpu.

Pretty much just blindly copy-pasted the existing generation code for capability except swapped it out for strings instead. Handily, we don't ever have to allocate a String, through the magic of &'static str.

[Jasper-Bekkers]

Looks alright - just to sanity check required_extensions can return several extensions, for example

             | BuiltIn::IncomingRayFlagsNV => &["SPV_NV_ray_tracing", "SPV_KHR_ray_tracing"],

And so the result of this should be interpreted as "either SPV_NV_ray_tracing, or SPV_KHR_ray_tracing" (both wouldn't make sense).

[khyperia]

yeah, it's really unclear to me in the spec whether there's an AND or an OR in some of these places - does OpPtrDiff require Addresses AND VariablePointers, or OR? so I just did the same ambiguous confusion that's already there and just copied in the JSON data directly, haha.

[Jasper-Bekkers]

I think it's OR in that case as well though it's not or badly documented in SPIR-V or their respective extensions.