Use `Ty` in kernel macros instead of Rust types

[mortendahl]

Related: https://github.com/tf-encrypted/runtime/issues/462

[mortendahl]

Note: one potential problem with this is that I don't know how we can statically go from Ty to type, and without that we loose the ability to statically check for kernel support.

One solution might be to have three sets of types (two of which generated by types!):

• the types we already have which represent runtime/compile values
• the Ty we already have which are serializable
• a new set of types which match Ty but are richer than the first set: TensorTy<Float64Dtype>, TensorTy<Float32Dtype> etc.