dest_varTerm.dest_var : term -> string * hol_type
Breaks apart a variable into name and type.
If M is a HOL variable, then dest_var M
returns (v,ty), where v is the name of the
variable, an ty is its type.
Fails if M is not a variable.
Term.mk_var, Term.is_var, Term.dest_const, Term.dest_comb, Term.dest_abs