Provide automatic backwards compatibility support

[edsko]

It is of course not possible in the general case to provide a single Haskell API (bindings) that can work with different versions of the underlying C library. However, one could imagine that for simple changes, there are things we can do:

• Provide default values for new parameters
• Do implicit conversions for simple type changes

etc.

[phadej]

This and other multi-version related issues need a diffing and merging support (not only figure out what's different, but also how to make a whole from two - or - more inputs). I'm afraid this can be come very tricky very fast.