Add more program context to json output

[waskyo]

This is known to be an issue with the json output, but perhaps it applies more widely.

Currently proofs only include a line number, which is not enough to disambiguate and pinpoint when multiple proofs are present in a single source line.

It is not clear to me if we need to do something special in the presence of macros?

[sipma]

Thank you for suggesting this addition to the json output!

Program contexts have now been added, with commit 9c7b666 .

A description and example are given here: #50 (comment)

[waskyo]

Leaving this issue open to also cover the investigation of adding column numbers to the program context.