[xla:ffi] Revised the list of supported attribute types and added tests
[xla:ffi] Revised the list of supported attribute types and added tests

Turns out the previous commit did not update the runtime, this the FFI failed
with an error when constructing a frame with e.g. bool attributes.

Here I decided to

* Only support signed integer attributes, i.e. leave out unsigned integer
  types until we have a use-case which needs them.
* Drop support for boolean arrays as attributes. The reasons are purely
  techincal: MLIR stores them as std::vector<bool> which has
  implementation-defined storage. Thus, we have to copy the vector to be able
  to expose it as XLA_FFI_Array.
