Ways to modify writeControls during writing

[shirok]

We want to tweak writeControls temporarily while write method is called recursively, e.g. we want to print a part of data structure with certain controls, but want to inherit other control options.

The controls have a dynamic extend of outermost write procedure, and its per port. Currently such transient controls are hidden from Scheme. (It is available in the printer method in C level).

How shall we expose such transient controls and allow users to modify it?

• Expose it as if it is a dynamic state (e.g. "current write controls"). It's per-port so it can't just be a parameter.
• Do not expose it, but allows users to "merge" given controls to the current implicit one. We need a special way to indicate "don't care" or "use the fallback" value to create a new controls.