specl: better timer resolution when luaposix is available.

* lib/specl/util.lua (gettimeofday, timesince): New functions to
wrap posix.gettimeofday and posix.timersub when available, or
else fall back to os.time and integer subtraction respectively.
* docs/specl.md: Document use of `util.timesince`.
* lib/specl/matchers.lua (M.stats): Use util.gettimeofday.
* lib/specl/formatter/progress.lua: Use util.timesince to display
elapsed time.
* lib/specl/formatter/report.lua: Likewise.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
refactor: move tmpfile handling to spec_helper.lua.

* specs/spec_helper.lua (Tmpfile): New Object derived tmpfile
wrapper.
* specs/specl_spec.yaml: Simplify accordingly.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>