Restore reserved names check (self_ast_imperative)

Problem

!1356 (merged) removed the self-cst passes in profit of self-AST passes but it forgot to add the old test for "reserved names" back (sorry o_o')

Solution

The check is back - in self-ast-imperative this time - and won't be replicated for all syntaxes

list :

    "unit"; "source"; "sender"; "failwith"; "bitwise_or"; "bitwise_and"; "bitwise_xor"; "string_concat"; "string_slice"; "crypto_check"; "crypto_hash_key";
    "bytes_concat"; "bytes_slice"; "bytes_pack"; "bytes_unpack"; "set_empty"; "set_mem"; "set_add"; "set_remove"; "set_iter"; "set_fold"; "list_iter";
    "list_fold"; "list_fold_left"; "list_fold_right"; "list_map"; "map_iter"; "map_map"; "map_fold"; "map_remove"; "map_update"; "map_get"; "map_mem";
    "sha_256"; "sha_512"; "blake2b"; "address"; "self_address"; "implicit_account"; "set_delegate"; "true"; "false";
    "assert"; "black2b"; "continue";  "gas"; "hash"; "hash_key"; "pack"; "sha256"; "sha512"; "stop"; "time"; "unpack";
    "continue"; "debugger"; "do";
    "get_chain_id"; "abs";
  ]

• has a changelog entry