Apply the first of the conversions in a given list that succeeds.
DESCRIPTION
FIRST_CONV [c1,...,cn] t returns the result of applying to the term
t the first conversion ci that succeeds (or raises UNCHANGED)
when applied to t. The conversions are tried in the order
in which they are given in the list.
FAILURE
FIRST_CONV [c1,...,cn] t fails if all the conversions c1, ...,
cn fail when applied to the term t. FIRST_CONV cs t also fails
if cs is the empty list.