Move CorpusSize to a new CorpusStats class.

This takes another mixin out of the ecosystem.  I'm really
migrating these in the wrong order, because I'm now needing some
::Solr scope qualifiers scattered about, but I'll fix that shortly.
Move Solr::Connection and SearchResult into namespace.

Also cleans up the term vector parsing, splitting it into the
ParseTermVectors class.