Make 'import tensorflow' go faster

It now takes about 400ms rather than 800ms, if the file system cache is warm.

Most of the latency was due to parsing text_format OpList protocol buffers in
our generated sources. We now use a binary representation, while preserving the
text proto as a comment for readability.

Note: This change does not improve the latency of dereferencing tf.contrib,
which takes about 340ms.
PiperOrigin-RevId: 163739355