message: Clean filter/check.rs, simplify some .map statements
diff: @@ -56,7 +56,7 @@ fn parse_expr<'a, T: 'static + Debug>(
  Some(_) => Ok(Parsed::StringRef(&ident.value)),
  None => filter_context
  .get_value(&ident.value)
  - .map(|value| Parsed::ValueRef(value)),
  + .map(Parsed::ValueRef),
  },
  Expr::CompoundIdentifier(idents) => {
  if idents.len() != 2 {
  @@ -68,7 +68,7 @@ fn parse_expr<'a, T: 'static + Debug>(
  filter_context
  .get_alias_value(table_alias, column)
  - .map(|value| Parsed::ValueRef(value))
  + .map(Parsed::ValueRef)
  }
  Expr::Subquery(query) => select(storage, &query, Some(filter_context))?
  .map(|row| row?.take_first_value())
  