F* cannot prove decidable equality with Hashmaps

[maximebuyse]

struct s2(std::collections::HashMap<i32, i32>);

Open this code snippet in the playground

This seems to be a regression from #1166. F* fails on the HashMap argument with:

  - Failed to prove that the type
    'Playground.t_s2'
    supports decidable equality because of this argument.
  - Add either the 'noeq' or 'unopteq' qualifier
  - The SMT solver could not prove the query. Use --query_stats for more
    details.
  - See also Playground.fst(6,0-12,11)

[maximebuyse]

Should be fixed by 6701b54