Support strings (&str, String, and Cow) in the argument position and String in the return position. Avoids
copies where possible, though this is not always something we can do.
Implementation of generics for `#[op2]`, along with some refactoring to
improve the ergonomics of ops with generics parameters:
- The ops have generics on the struct rather than the associated
methods, which allows us to trait-ify ops (impossible when they are on
the methods)
- The decl() method can become a trait-associated const field which
unlocks future optimizations
Callers of ops need to switch from:
`op_net_connect_tcp::call::<TestPermission>(conn_state, ip_addr)` to
`op_net_connect_tcp::<TestPermission>::call(conn_state, ip_addr)`.
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.
