KIP-0029: Proposal of new guard type: Constant boolean guard

[CryptoPascal31]

The idea is to propose a new guard, the most simple as possible, the easiest to understand.

But it's currently missing, and would be very useful, and would improve the clarity. Nothing revolutionary.

https://github.com/CryptoPascal31/KIPs/blob/119fe2df7df6a42bba7f46c406ca631aec95e99a/kip-0029.md