Full API support for configuration

To-do list from the design sketch:

• Sketch out new toml file hierarchy and option names.
• One of these:
  • Remove dependency from arti-client to tor-config. (Perhaps by using directories crate directly).
    • Then, optionally, rename tor-config to arti-config, on the theory that higher-level crates get called arti-foo.
  • Split tor-config crate in two, one with low-level and higher functionality and one with high-level functionality.
• Lower top-level configuration logic from arti to the ??-config crate.
• Rename options and adjust hierarchies to match the new toml hierarchy and option names.
• Implement the meta-builder pattern for ArtiConfig (the one in ??-config)
• Implement the meta-builder pattern for TorClientConfig (the one in arti-client)
• Implement builder reconstruction for ArtiConfig
• Implement builder reconstruction for TorClientConfig
• Implement by-name TOML mutability for arti-config crate.
• Implement reconfiguration for TorClient and related structures.