Have ArtiConfig contain a TorClientConfig, and drop builder retcon

Replace the recapitulation of TorClientConfig fields in ArtiConfig and
instead just have it contain one.  This is part of #374.

The conversions from ArtiConfig back to ArtiConfigBuilder and
TorClientConfigBuilder would need to change, but, since we don't want
them anyway,

No longer impl Deserialize for ArtiConfig.  (As per #371 this will
want to become a private type.)

No longer impl From<ArtiConfig> for ArtiConfigBuilder and
TorClientConfigBuilder.  And abolish tests of that code.

(This all has to be in one commit, because previously
ArtiConfig::tor_client_config used the validated-to-builder config
retcon.)