[meta] const/override-expressions

[teoxoy]

const-expressions phase 1 (IR architecture changes) gfx-rs/naga#2266:

• [wgsl-in] Rejection of matrix consts naga#1956
• [wgsl-in] Rejection of composite consts without explicit nested types naga#1957

const-expressions phase 2 (const evaluation + support for const-expressions in remaining places (attributes and switch cases)) gfx-rs/naga#2309:

• Constant propagation naga#97
• [wgsl-in] Implement const expressions naga#1829
• [wgsl-in] Support const operations in const variables naga#733
• Cannot define fractional constants naga#471
• workgroup_size: expected 32-bit signed integer literal error naga#2430
• Support Constants in Enum Cases naga#2106
• [wgsl-in] 0.0 / 0.0 should not be allowed naga#2313

override-expressions:

• Pipeline-overridable constants #4366 (+ Pipeline overrides #1762)
• Support override-expression for workgroup_size #4450
• Allow override-expressions in element count of fixed-size arrays  #5315

full abstract types:

• [wgsl-in] Implement automatic type conversions (abstract types) #4400
• [wgsl-in] Incorrect handling of negative integers with hex values #4492
• [wgsl-in] Parse error for subtraction in array index #4435

misc (future work):

• Function-local const declarations not implemented #4493
• [meta] Constant evaluator built-in functions support #4507