refactor codes
add verbose flag
add 17xx for typechecker
rename T_LinkForward -> T_Link and T_ReferenceForward -> T_Ref
define func map_of & enum_symbol; dump environment/map when verbose; redefine func well_formed; pass environment/map when invoke func error
Invoke func well_formed after 3rd-pass. Evaluate t_explicit to ensure the type does exist. Type inference for symbol. Pass environment/map when invoking error.