Prevent unlinking for tophats with no wearers

[spengrah]

This PR prevents linked tophats that have no wearer from being unlinked, plus a couple other small QoL additions.

• [x] prevent linked tophats with no wearers from being unlinked
• [x] add several hat property getters (i.e. view functions)
• [x] expanded documentation for linking and unlinking

[topocount]

@spengrah You've probably considered this, but maybe if the local tophat of the tree to be pruned isn't worn by an eligible user, the failsafe could be passing in a wearer of The Root Tophat and minting a local tophat to that address.

Just an idea for a secure way of eliminating that extra step. The increased complexity might not be worth it though.