tor-keymgr: Initial implementation

This is the first-draft implementation of #857 (closed) and #859 (closed).

Limitations:

• these APIs are experimental and likely to change
• there are no tests
• the tor-keymgr::Error types aren't fully fleshed out yet
• there are still some outstanding questions that will need to be answered before we can declare the API stable (most of them can be found in the TODOs sprinkled throughout the code)
• the code isn't as polished as it could be, and it could use more documentation

Let me know if you see anything fundamentally wrong with this approach. Any other feedback is welcome, as are any suggestions for improving what I have so far.