Introduce scalars (bool/int/float) as "native" SPIR-T types, consts and runtime ops.

[eddyb]

Note: this PR is a draft to avoid accidental merging onto its "base" branch (used as a form of ad-hoc PR stacking), and will remain as such, until its "base" branch can be set to main, i.e. all prerequisite PRs will have landed, up to and including this PR (whose branch is the "base" of this one):

• Add a Vulkan layer for injecting SPIR-V -> SPIR-T -> SPIR-V into any Vulkan users. #41

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Originally submitted as (description mostly copied from there):

• Introduce scalars (bool/int/float) as "native" SPIR-T types, consts and runtime ops. EmbarkStudios/spirt#50

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This is the first big step towards:

• Bring many basic types/insts into SPIR-T (instead of leaving them as SPIR-V). EmbarkStudios/spirt#1
  (TODO: migrate the old issues)

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This is what the README.md example looks like now:

TODO: changelog (don't forget to mention the new switch! or should that be its own PR?), better docs, set up constant-folding (maybe even some OpSpecConstantOp support?).