Implement lowering 'decnet'

Examples:

- decnet src [host] <decnet-address>.
- decnet dst [host] <decnet-address>.
- decnet host <decnet-address>.
Add examples 'decnet src 10.15' 'decnet host 10.15'
Merge pull request #67 from Igalia/low-decnet

Implement lowering 'decnet'