Consider renamings for Arti 1.1.0

We have a few "TODO pt-client rename this!" items that we should consider before 1.1.0.

(Please don't do any of these renamings without discussion: Step 1 is to agree on a better alternative)

The offending identifiers are...

From tor-linkspec:

• DirectChanMethodsHelper (could become HasDirectChanMethodOnly?)
• ChannelMethod (maybe ContactMethod or RelayContactMethod?)
• BridgeAddr (once PtTargetAddr, renamed in !874 (merged))

tor-chanmgr:

• TransportRegistry
• TransportHelper

tor-guardmgr:

• for_circuit_usage