Design standard library API

**Context**: the stdlib API is currently in flux:

- PR #305 introduces some helpers for creating linear and affine (linear + bias) layers.
- It became clear that these and some other functions in `stdlib/nn/mod.rs` can't easily be expressed as `Definitions`
- Deciding what goes in stdlib and what its public interface should be is not clear

**Task**:

- Fix a public API for all the exports from the stdlib module
- Pick a rule (e.g. "Every layer in nn should be expressible as a `Definition`") for what is included
- Doc comments + autogenerated diagrams for each definition / function