maint: rename lib/specl.lua to lib/specl/runner.lua.

* lib/specl.lua: Move from here...
* lib/specl/runner.lua: ...to here.
* lib/main.lua: Adjust require statement accordingly.
* local.mk (dist_noinst_DATA): Adjust.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specl: filter examples by line-number with +NN argument.

* specs/specl_spec.yaml: Specify behaviour of +NN argument.
* specs/spec_helper.lua (Tmpfile): Add __tostring metamethod to
support new specl_specl.yaml specs.
* lib/main.lua (filters): Store +NN filters in advance of
following FILE.
(process_args): A variation of std.io.process_files that saves
+NN filter to `filters`.
(slurp): Rename from this...
(compile): ...to this.  Collect line filters into specs table.
* lib/specl/runner.lua (run_examples): If any filters were
set, then check only the examples from this file with matching
line numbers.
* lib/main.lua (OptionParser): Update spec string to describe
use of +NN filters.
* NEWS: Update.

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