Replace a use of div with quot

This actually makes about a 7% difference in performance!
Replace countDigits with a faster, more complex version

This is taken from Andrei's "Three Optimization Tips for C++" post:
https://www.facebook.com/notes/facebook-engineering/three-optimization-tips-for-c/10151361643253920

It improves performance by up to 15%.
A more straightforward (and slower) countDigits

This is a few percent slower than the tail-recursive version for
numbers of more than one digit.
Backed out changeset bb9a0e19421e, since it was slow
Benchmark some bigger numbers

--HG--
extra : amend_source : b9fc539b407b29187bf818d89f4d997dfbbb36f6
Switch to a faster decimal algorithm

This is about 25% faster than its predecessor for large numbers.

--HG--
extra : amend_source : 9917efd5e13a7284857236e9162614a0731e0636
Backport integer builder benchmarks
Merge
Replace uses of quot and rem with quotRem

Astonishingly (at least to me), this improves performance by almost
30% for large integers.
Shave off another 6ns for negative integers with quotRem
Compare Show instance performance
Handle Int8 overflow
Undo an overflow bug I introduced with quotRem
Replace the few last uses of div with quot
Fix printing of hex Integers (hg-47)