feat: expose contenthash helper methods

[bguiz]

What

• src/contenthash-helper.ts contains encode and decode methods --> expose these under rns.utils.

Why

• Code which uses rns.js as a dependency may need to convert between the two forms, because:
  • rns.contenthash(name) (getter) returns in decoded form
  • rns.setContenthash(name, contenthash) (setter) has parameter in encoded form
  • do own assessment of validity, by catching errors thrown by these helper function