Leverage constexpr and __builtin_ffs for Bitmap::find_first. Also add
a variant Bitmap::find_one_anywhere that can start scanning at a
provided hint.
Also, merge Bitmap::fill_range into the already existing Bitmap::set_range
When we switched the Bitmap code to operating 32 bits at a time,
we neglected to look in the trailing remainder bits after the last
full 32-bit word.
This patch fixes that and adds a couple of tests for Bitmap that I
hacked up while tracking down this bug.
I found this bug when noticing that the kernel would OOM while there
were still some pages left in the physical page allocator.
