Sorts a list of variables according to first argument.
DESCRIPTION
A call to sort_vars [s1,s2,..sn] vs will return a permutation of
vs such that variables with the name s1 will appears first,
followed by those with the name s2 etc.
FAILURE
Never fails.
EXAMPLE
> sort_vars ["a", "b", "d"] [``b:bool``, ``c:num``, ``d:bool``, ``a:'a``];
val it = [``a``, ``b``, ``d``, ``c``] : term list