Add explicit `?` to Line syntax

[bkushigian]

Currently we have line strings of the form r:0:c:b18:c:c, and there is an implicit turn card in the middle that we want to fill in. I currently need to do some basic state machine stuff to identify where that is. We should add logic to explicate the ? into the line string.

• Upside: more readable, easier to substitute cards in (e.g., when iterating over all nodes that correspond to the line)
• Downside: more expensive to make this form by default (string operations whenever we create a Line)

Should think about this a bit. Maybe add a cached explicit_line_str or something that we can populate when we first need it?