In pattern compilation, we only need to know the full list of
constructors for a given type, whenever
1. we want to check that a symbol used in a pattern is indeed
   a constructor;
2. we want to check for non-exhaustive matching and return an
   example of a non-covered pattern, if any.
Thus, we need to give Pattern.compile access to the current
known_map whenever we check new declarations in Decl or Mlw_decl.
However, once we have checked the patterns, we do not need the
full constructor lists just to compile the match expressions.
Just knowing the number of constructors (provided in ls_constr)
is enough to detect non-exhaustive matching during compilation.