include: clean up includes in uv.h

* size_t is defined in <stddef.h>, not <sys/types.h>

* Simplify the platform check.
include: remove UV_IO_PRIVATE_FIELDS macro

It's an alias for UV_IO_PRIVATE_PLATFORM_FIELDS.  Remove the macro and
use UV_IO_PRIVATE_PLATFORM_FIELDS directly.
include: fix typo in comment in uv.h
include: update uv_is_active() documentation

Seems to have been overlooked in the reference counting refactor back
in May 2012 in commit 9efa8b3.  Clarify what "active" means for
different kinds of handles.
include: make uv_process_options_t.cwd const

There is no reason for it to be non-const, libuv treats it as a
read-only input.  Make it a pointer to a const string.
test: wrap long lines at 80 columns
unix: wrap long lines at 80 columns
unix, windows: make uv_is_*() always return 0 or 1

Ensure that the following API functions always return either 0 or 1:

  * uv_is_active()
  * uv_is_closing()
  * uv_is_readable()
  * uv_is_writable()
bench: measure total/init/dispatch/cleanup times

* Add finer-grained measurements to the million_timers benchmark.
  Before this commit it only measured the total running time.

* Bump the number of inserted timers from 1M to 10M.  With one million
  timers, it finishes too quickly to get useful profiling data out of
  it with perf or oprofile.  The name of the benchmark is now a lie but
  such is life.