Expose chain ID in Clarity

[MarvinJanssen]

Having a native constant for the chain ID in Clarity is very useful for various reasons. One can build triggers based on mainnet/testnet, etc., but it is specifically useful for signed message verification. A common way to prevent replay protection across forks is to add a chain ID to the message somewhere. Right now that means you have to hardcode the chain ID.

The chain ID is expressed as 4 bytes in a transaction and as a u32 in Rust:

• 0x00000001 or 1 for mainnet
• 0x80000000 or 2147483648 for testnet

My suggestion is to expose it as chain-id with a type of either uint or (buff 4).

[jcnelson]

This has been merged to next.