Implement a new struct btype (inherits from struct type) with id BITFLD, to contain...

Implement a new struct btype (inherits from struct type) with id BITFLD, to contain the struct FS (field specification, i.e. flen and bitoffs), in a similar way to how struct stype (inherits from struct type) with id STRUCT contains the struct SS, replace the nl_flags&FFIELD test with a new nl_dtype->t_id==BITFLD test