Errors should have multiple locations, not just one

[cpitclaudel]

This should be an easy change, though it touches a bunch of places in the code. The idea is that some errors come with multiple ranges, and these should all be reported to the user. The interactive mode already accepts lists of ranges, so no changes would be needed on that side.

Currently, F* reports auxiliary locations as strings ("see also …", "Also see …"). It would be much nicer to return these locations as lists of ranges.

This was brought up by @fournet, who noted that being able to jump between errors and related locations would be convenient.