If ty is a base type, then dest_type t returns ("ty",[]).
If ty is a compound type (ty1,...,tyn)tyop, then (dest_type ty}
returns (tyop,[ty1,...,tyn]).
EXAMPLE
- dest_type bool;
> val it = ("bool", []) : string * hol_type list
- dest_type (alpha --> bool);
> val it = ("fun", [`:'a`, `:bool`]) : string * hol_type list