This should probably be abstracted into a macro to define UL vs ULL on
the given platform based on sizeof(void*).

This should be a bit easier to maintain to boot. I'm removing the
per-platform swapping functions since they are mostly moot anyway.
Ideally, we would add support for non-GCC builtins in the future.
Clang and Intel compilers seem typical.

Our bson_iter_t will catch this problem, but since it is a constant
time operation it seems useful enough to do upfront.

This commit initializes using /dev/urandom as well as printing the value
to stderr for use in replicating results. Useful for making sure the
fuzzer always tests with new data but allowing recreation of failure.

bson_data() is rather slow when used in a hot path. Just call it once
and increment our buffer pointer manually. This is safe because we have
already done our buffer growth outside this hot path.

Let the compiler setup the constant mask directly.

This is actually a pretty healthy performance boost. We avoid
traversing the hierarchy which means we also don't pollute the cache
lines so much.

If we just initialize everything to the default, we can just use malloc
instead of calloc for a bit of a speed improvement.

The default pointer alignment is fine since we only care about cache
alignment for stack allocations using inline structures.