Parse a string into a significand and exponent according to the
"Hexadecimal Floats in Haskell" proposal. A trivial example might be:
ghci> readHexSignificandExponentPair "0x1p+1" (1,1) Behaves similar
to readSignificandExponentPair but the base is 16 and numbers are
given in hexadecimal: ghci> readHexSignificandExponentPair "0xAp-4"
(10,-4) ghci> readHexSignificandExponentPair "0x1.2p3" (18,-1)