Use a text field for taxonomy term synonyms

Very much needs work, but CNR for the test bot.

Moves synonyms to a text field, started very basic and broken conversion of synonym related code (since synonyms collapsing went into D7 we either need to support that or rip it back out again) - issues with that is the current code joins on the synonym table, with pluggable field storage and various other hooks, we don't even know where the synonyms might get stored. Upgrade path is more or less there though.