tvix: fix GC escapes

Currently, attempting to run tvix with the GC enabled results in spectacular crashes because various pointers in libexpr are not traceable, leading to use-after-frees during evaluation.

Known GC escapes:

• nixexpr.hh: AttrPath - std::vector<>
• nixexpr.hh: AttrName - std::variant<>
• parser.y: union YYSTYPE - is a union

1. GC is gone.

   riking at 2020-08-17T01·33+00

2. riking closed this issue at 2020-08-17T01·33+00