reorder statements in timeouts_get to make TIMEOUT_DISABLE_INTERVALS case simpler, removing need for macro accessors

Merge branch 'optional_intervals' of git://github.com/nmathewson/timeout into nmathewson-optional_intervals

Merge branch 'optional_timeout_ptr' of git://github.com/nmathewson/timeout into nmathewson-optional_timeout_ptr

Merge branch 'timeout_foreach' of git://github.com/nmathewson/timeout into nmathewson-timeout_foreach

Merge branch 'timeout_cb_override' of git://github.com/nmathewson/timeout into nmathewson-timeout_cb_override

Merge branch 'portable_bitops' of git://github.com/nmathewson/timeout into nmathewson-portable_bitops

According to the documentation, 32-bit x86 builds do not have the
64-bit _BitScan{Forward,Reverse}64 intrinsics.

Given these, we can remove the code to check for the zero case.

This patch includes an intrinsic implementation for these functions
when the compiler is GCC, Clang, or MSVC--or if the compiler claims
to be one of those.  Otherwise, a naive implementation is used.

Tests are included for all of these functions, which turned up a
possible problem: according to the gcc documentation, __builtin_ctz
and __builtin_clz give an undefined result when their inputs is
zero.  I was not able to persuade myself that we always called them
with a nonzero argument.

This field eats a pointer for every struct timeout, and it only
exists in order to enable a set of APIs that omit the 'struct
timeouts' object. For some applications, the convenience of these
APIs is not worth the memory overhead. This patch makes it so
that applications can disable those APIs, and save some memory, by
defining TIMEOUT_DISABLE_RELATIVE_ACCESS at compile time.

Some users only need plain timeouts.  They shouldn't have to pay the
overhead for a field they will never set.  With this patch, they can
define TIMEOUT_DISABLE_INTERVALS and turn off the feature at compile
time.

Libevent needed this, and I expect that others will too.

I've found that frequently a void function and a pointer are either
more or less than we want to attach to each timeout object.  This
patch makes it so that the user can define TIMEOUT_CB_OVERRIDE to
indicate that they have included their own definition of struct
timeout_cb.

remove main() testing code from timeout.c and tweak Makefile so we can compare different wheel sizes with the benchmark framework

re-add one of our old bit fill operations, as it turns out it was actually needed (probably shouldn't have worked on that code in a sleep-deprived state)