The design doc for llook/rlook was incorrect, swapping the semantics per the updated docs and swapping argument order for llook.

Values cleanup and doxy: remove some redundancies, make the Begin/End constants in Position an enum, change tag_ref to contents

The agreement stuff isn't implemented yet.

- 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.

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

For example, it can now be used in Conditional, which is done as well.