[interpreter] Source locations are broken since menhir switch

[rossberg]

I only noticed now that the switch to menhir in #1705 completely broke the implementation of source locations for the AST – they are all empty, producing useless messages for type or runtime errors.

The current implementation depends on reading out Parsing.symbol_start/end_pos, which are defunct with menhir.

Unfortunately, menhir offers no easy substitute – afaict we'll need to change every single rule to use $sloc and $loc($i). Or revert #1705.

@zapashcanon, can you please look into this?

[rossberg]

Never mind, I fixed it: 5b26fa2

[zapashcanon]

Oh, sorry about that. Thanks for the quick fix.