[WIP] Introduce vectors as "native" SPIR-T types.

[eddyb]

This is part of:

• Bring many basic types/insts into SPIR-T (instead of leaving them as SPIR-V). #1

See also:

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

TODO: changelog, docs.

[eddyb]

Superseded by:

• [WIP] Introduce vectors as "native" SPIR-T types. Rust-GPU/spirt#43