Show panic! messages via debugPrintf, even including some runtime arguments ({u,i,f}32 as {} or {:?}).

[eddyb]

Also, this PR moves us in the direction of "preferring fewer optimizations" (we now force -Zinline-mir=off),
as that makes it easier to e.g. reliably pattern-match on the generated SPIR-V.

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By adding some out of bounds indexing to the sky-shader, and running:
VK_LOADER_LAYERS_ENABLE=VK_LAYER_KHRONOS_validation VK_LAYER_ENABLES=VK_VALIDATION_FEATURE_ENABLE_DEBUG_PRINTF_EXT DEBUG_PRINTF_TO_ STDOUT=1 cargo run -p example-runner-wgpu --release -- --force-spirv-passthru, we get:

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Note that bounds-checking panics come from a core function which happens to do:

panic!("index out of bounds: the len is {len} but the index is {index}")

(that is, we don't special-case bounds-checking panics, and simple Display/Debug formatting of 32-bit integers/floats, including usize/isize, should work, and we can do a lot more in the future)

TODO: changelog.