Add siac utils

With support for offline transaction signing coming, power users will be doing more manual construction of transactions. siac utils is a new family of subcommands that provides various helpers for converting transactions to JSON or base64, converting currency values to Hastings, calculating SigHashes, verifying signatures, and more.

siac utils will not necessarily be limited to transaction-related helpers; the siac commands for generating man pages and bash completions have also been moved under the utils umbrella.