Rust version of sshnpd

[cpswan]

Is your feature request related to a problem? Please describe.

The python version of sshnpd has substantial underlying requirements that use a lot of flash storage.

Customers are asking for something that will fit into ~2MB Flash on a MIPS based OpenWRT device.

Describe the solution you'd like

A Rust based version of sshnpd that will fit into 2MB by making use of existing shared libraries for crypto etc.

Describe alternatives you've considered

C would also fit in well with existing OpenWRT packages.

[cpswan]

During PR73 we've been able to look at Rust (and the Rust tooling within OpenWRT) closely enough to establish that a Rust version is unlikely to satisfy the needs expressed. So shifting focus to C, and moving this to the backlog.

[cpswan]

Dropping this after considerable progress and learning with C version.