grok_bin_oct_hex: Minor improvements to precision, speed

[khwilliamson]

These commits

• add a calculation before the loop that saves calculations each iteration
• replace floating multiplications each loop iteration with an integer increment, plus after the loop a call to ldexp

One question I have is can I just assume that Perl_ldexp exists and not have a fallback to Perl_pow? I think C99 requires all versions we need to be available, but it's not clear to me that perl.h guarantees one.

• This set of changes does not require a perldelta entry.