Conversion to perform exponentiation on a integer literal of type :int.
The call INT_POW_CONV `c pow n` where c is an integer literal of type
:int and n is a numeral of type :num, returns |- c pow n = d where d
is the canonical integer literal that is equal to c raised to the nth
power. The literal c may be of the form &n or -- &n (with nonzero n in
the latter case) and the result will be of the same form.
Fails if applied to a term that is not a permitted integer literal of type
:int raised to a numeral power.
# INT_POW_CONV `(-- &2) pow 77`;;
val it : thm = |- -- &2 pow 77 = -- &151115727451828646838272