- since the default value for Position is Nowhere now.
Also, remove reduntant header and change stream to ostream in to_strings

The concept has been reworked a bit.

It is called "flex" atm but it is subject to change.

Also, remove redundant goto_starts

It was trying to parse a position instead.

Add the tagset-symbol-getter operator (GetSymbols), bump required Corpus2 version to 1.0.2 since new features are needed.