Switch to fixed-size pegout hashes

Currently, we support 4-42 byte pegout addresses, which are included in the kernel. The same 4-42 bytes also make up the scriptPubKey of the pegout output in the HogEx transaction. So we're duplicating it each time.

We should instead just include the 20-byte hash of the scriptPubKey in the kernel to avoid that duplication.

No files or tests are named. Start by tracing how pegout addresses are represented in the kernel and in the HogEx transaction; confirm the current 4–42-byte data path. Done means the kernel uses a fixed 20-byte scriptPubKey hash and no longer duplicates the full pegout data.

Written by the indexing model from the issue text.