code reorganization

[borg323]

The src directory is re-organized as follows:

1. The benchmark and lc0ctl directories are merged into tools.
2. All backends are moved into neural/backends.
3. The neural/onnx and neural/xla directories remain but only contain the conversion functions and not any backend code.
4. The neural/tables directory contains some generic information that used to be in neural/shared (activation functions, policy maps etc.) that are used by both backends and the onnx converter.
5. The rest of neural/shared is moved to neural/backends/shared/.
6. The rescorer is moved into the trainingdata directory.