Here's an updated patch that makes the trailing 'p123' exponent optional 
in fromhex.  (This matches the behaviour of C99's strtod and sscanf;  in 
contrast, Java always requires the exponent.)

I'm beginning to wonder whether the '0x' shouldn't also be optional on 
input as well, in the same way that it's optional in int():

>>> int('0x45', 16)
69
>>> int('45', 16)
69

This would then allow, e.g.,

>>> float.fromhex('45')
69.0