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CI: Fix for changed dylib extension on Tensorflow on darwin
Fix compatibility with older Rust versions

Update dependencies
Bump version to 0.6.0
Add fix suggested by clippy
Mark trait objects with dyn

Rust 1.37.0 gives warnings for bare trait objects.
shell.nix: add Tensorflow GPU improvements

- Override arguments of libtensorflow_1_14_0.
- Bump danieldk package set for Tensorflow with compute capabilities
  3.5, 5.2, and 7.5.
Nix: bump danieldk package set for fixed TF 1.14 on Darwin
Fix byte encoding

tf.strings.unicode_decode decodes to unicode codepoints. So, any non-latin-1
character falls outside the range. This change introduces actual byte

In some experiments, I found that with real byte representations, it helps
to have two layers in the byte RNN (presumably to combine characters with
multi-byte encodings), so this also changes the default hyperparameters.
Add support for RNN-based subword representations

When subword representations are enabled, sticker builds a string
tensor containing each form in the input. The forms are converted to
(truncated) byte representations in the Tensorflow graph. Each byte is
then mapped to a byte embedding. Subword representations are then
formed by applying a bidirectional RNN to the sequence of byte

In order to support the configuration of subword embeddings, the
sticker configuration format has been adjusted. An `input` section has
been added, which holds the new `subwords` boolean option. The
`embeddings` section is now a subsection of `input`. The use of
subword representations is communicated to the graph writers through
the shapes file.
Add references to papers that inspired sticker
Bump version to 0.5.1
Rename tf-proto to sticker-tf-proto
Improve encoder error handling
Restructure encoder modules

- The top-level depparse module is renamed to encoder::deprel.
- The Layer{Encoder,Decoder} are moved to the encoder::layer module.
CI: update to Tensorflow 1.14
CI: copy and doc/ on deploy
Bump version to 0.5.0
Store graph construction args to the graph

Also provide the sticker-graph-metadata utility to print the args.