Tensor bundle: a module to efficiently serialize/deserialize tensors.

This module will serve as the backing store of TensorFlow's V2 checkpoint
format.

Highlights of the benefits:

* Peak memory usage of both write and read paths are just 1x (plus a small
  constant-sized buffer).
* Stores tensors in a non-proto format, circumventing the size limit on proto
  messages.
* Flexibility: potentially allows by-row loading of a large tensor, or mmap.
Change: 134028119