Major Features and Improvements
* Added `tff.types.tensorflow_to_type` function to convert structures
containing tensorflow type specs into a `tff.Type`.
* Deprecated `tff.types.infer_unplaced_type`.
* Updated `tff.types.ArrayShape` to be defined as a `Sequence` not an
`Iterable`, this is because the `len` of an `tff.types.ArrayShape` is used
for comparison.
* Deprecated the `type_signature` parameter for the
`tff.program.ReleaseManager.release` method.
Breaking Changes
* Removed the implementation of `tff.Value.__add__`.
* Removed the deprecated `tff.Type.check_*()` functions, use `isinstance`
instead.
* Removed `tff.types.at_clients` and `tff.types.at_server` functions, use the
`tff.FederatedType` constructor instead.
* Removed support for handling `tf.data.DatasetSpec`, `tf.RaggedTensorSpec`,
and `tf.SparseTensorSpec` using `tff.to_type`, use
`tff.types.tensorflow_to_type` instead.
* Removed support for handling `tf.RaggedTensor` and `tf.SparseTensor` using
`infer_type`.