* add feature for height locked kernels
* add function to compute kernel features appropriate for lock height, and use it
* only sign kernel-features relevant fields; refactor Features
* simplify invalid kernel logic
* remove unused height arg to reward::output and run some rustfmt
* replace nested if/else by match
* include kernel features in msg being signed
hash the msg before signing it (for consistent 32 bytes)
* rustfmt
* fix various tests
* no HF for this (mainnet only)
