Support SPV_KHR_8bit_storage

- Add asm/dis test for SPV_KHR_8bit_storage
- validator: SPV_KHR_8bit_storage capabilities enable declaration of 8bit int

TODO:
- validator: ban arithmetic on 8bit unless Int8 is enabled
  Covered by https://github.com/KhronosGroup/SPIRV-Tools/issues/1595