Scalar and Vector types refactor

[Firestar99]

Breaking changes

• rename VectorOrScalar to ScalarOrVector to match spec expression used in subgroup intrinsics
• changed publicly exported paths
  • spirv_std::vector::VectorOrScalar -> spirv_std::ScalarOrVector
  • spirv_std::scalar::Scalar -> spirv_std::Scalar
  • spirv_std::vector::Vector -> spirv_std::Vector
  • spirv_std::number::Number -> spirv_std::Number
  • spirv_std::integer::Integer -> spirv_std::Integer
  • spirv_std::integer::SignedInteger -> spirv_std::SignedInteger
  • spirv_std::integer::UnsignedInteger -> spirv_std::UnsignedInteger
  • spirv_std::float::Float -> spirv_std::Float
• these are probably barely used outside of spirv-std directly

Changes

• move Scalar traits (Number, Integer, SignedInteger, UnsignedInteger, Float) to mod scalar
• simplify proc macro to declare Scalars and Vectors
• move VectorOrScalar and Sealed to new mod vector_or_scalar
• remove root-level mods integer, number, sealed
  • keep float since it has intrinsics

[eddyb]

Looking at this now, they could've been in a spirv_std::traits module from the start, but the consolidation from this PR is already most of the improvement.