feat: rif name util

[bguiz]

What

• impl rns util + demo test
• registers a new name if necessary, using FIFSAddrResolver, via makeCommitment and register (via RIF token's transferAndCall)
• if necessary, calls setResolver to upgrade to "definitive resolver"
• if necesary, calls setAddr again subsequent to setResolver
• finally, call setContenthash to either set or update domain to new content

Why

• add a code-only means to register a .rsk name
  • currently manually using a browser to interact with the rns manager dapp

Refs

• Much thanks to @javiesses for helping me to understand the internals of resolver and contenthash
• feat: impl rns.resolver(name) - getter function rnsdomains/rns-js#122
• feat: expose contenthash helper methods rnsdomains/rns-js#123
• Add .resolver(domain) method rnsdomains/rns-js#121