Add builder for encoding hidden service descriptors.

• Add builder for encoding hidden service descriptors.

  This introduces the NetdocBuilder trait described in netdoc-builder.md and a new tor-netdoc::doc::hsdesc::build module, which exports the HsDescBuilder. Hidden services will use HsDescBuilder to build and encode hidden service descriptors.

  There are several TODOs in the code that I'm planning to address separately.

  Partially addresses #745 (closed).

• Define constants for AuthClient field lengths.

  These are used in multiple places (and will also be used by the HS descriptor encoder later on), so let's make them named constants.

• Implement ItemArgument for String.

  This makes some code a bit more concise, as it allows us to make Strings into ItemArguments without calling .as_str().