This is a new op system that will eventually replace `#[op]`.
Features
- More maintainable, generally less-coupled code
- More modern Rust proc-macro libraries
- Enforces correct `fast` labelling for fast ops, allowing for visual
scanning of fast ops
- Explicit marking of `#[string]`, `#[serde]` and `#[smi]` parameters.
This first version of op2 supports integer and Option<integer>
parameters only, and allows us to start working on converting ops and
adding features.
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