DatumInContext.__repr__ is offputtingly verbose

[jnothman]

For someone playing around with the library in an interactive environment, running some_path.find(some_obj) will return a list of DatumInContexts each of which will be formatted on screen, including the recursive context for every element. Hiding the context avoids large outputs and user confusion. I would rather see one of the following:

• DatumInContext('value')
• DatumInContext('value', path='path')
• DatumInContext('value', path='path', context=...) or DatumInContext('value', path='path', context=None)
• DatumInContext('value', path='path', context=DatumInContext(...)) or DatumInContext('value', path='path', context=None)

than the fully expanded context repr.