variable-length-quantity: no-op declare compliance with 1.2.0
variable-length-quantity: no-op declare compliance with 1.2.0

JSON now uses error object to indicate "incomplete sequence" rather than
null.

Rust has and will continue to translate this to `Err`; no action
needed.

https://github.com/exercism/problem-specifications/pull/1337

petertseng/exercism-rust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,exercism/xrust