Add proper implementation of next_clear_bit for Bitset and update int_al...

...locator to use it, and fix issues with Bitset creating large fixnums.

- Fix lots of left-shift related bugs in Bitset (as we were creating a large number of BigNum's with potentially thousands of binary digits to hold our "words" due to left-shift converting FixNums to BigNums. It's still going to happen from 61-64 bits, but way better than 2048-bit fixnums being used).
- Added benchmark for IntAllocator. It used to take up to a minute to allocate 10,000 integers on a 2.7 GHz Intel Core i5, now takes 4 seconds for 65,536 (which we killed after waiting for 6 minutes using the old allocator).