Tracking issue for full-feature inline

[jbachurski]

inline is currently missing:

• [x] Support for subgraphs (they should get prefixed properly, which ONNX renaming doesn't do)
• [ ] Support for functions (such local/dependency functions should be propagated to the built model)
• [x] Capability not do add extraneous Identity nodes for all model input/outputs (requires better renaming capabilties)

Most of these can be fixed by reimplementing renaming values in graphs, as the ONNX feature (onnx.compose.add_prefix_graph) is rather lacking.