consistently use 'loc' instead of 'range' to describe source locations

The problem with 'range' is that this word has a much more general
meaning, it could be an integer range; typically it is also used in
comments to describe range of levels or scopes.

In the context of Menhir, which is centered on parsing, 'range'
clearly associates with 'source range'. But for more general project
this association is lost and 'range' is confusing.