Support Roughtime from Rust

[daviddrysdale]

We currently have some Roughtime client code in C++ (#428, #461, #468). It's not yet plumbed into the runtime (there's no ABI to expose it yet), but it will eventually need to be accessible from the Rust-based version of the runtime (#723). That could either be:

• because it's possible to link C++ functionality into the Rust runtime (Support building and linking C/C++ code to Rust oak_loader #724), or
• because we have a pure Rust implementation of the Roughtime client.

[daviddrysdale]

As of #845 and #801 the Rust runtime can use the C++ Roughtime client pseudo-Node via FFI.

[conradgrobler]

#1047 adds a Rust version of the c++ Roughtime client to the runtime. The pseudo-node will follow.