specs: no need for --unicode any more.

* specs/specs.mk (SPECL_OPTS): Remove --unicode.  There are no
spec files with unicode in them in the project any more.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specl: prefer new `to_` syntax over `should_` for expectations.

* specs/custom_formatters_spec.yaml, specs/environment_spec.yaml,
specs/formatter/report_spec.yaml, specs/matchers_spec.yaml,
specs/shell_spec.yaml, specs/specl_spec.yaml: Update to `to_`.
* specs/should_spec.yaml: New specs to ensure `should_` continues
working.
* specs/specs.mk (specl_SPECS): Add specs/should_spec.yaml.
* lib/specl/matchers.lua (expect): Accept new `to_` syntax.
* doc/specl.md: Update.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
matchers: accept `not_to` as an alias of `to_not_`.

* specs/custom_formatters_spec.yaml, specs/matchers_spec.yaml,
specs/shell_spec.yaml, specs/specl_spec.yaml,
specs/formatter/report_spec.yaml: Use `not_to_` in preference to
`to_not_`.
* specs/matchers_spec.yaml: Add some `to_not_` examples to ensure
they keep working.
* lib/specl/matchers.lua (expect): Accept `not_to_` as an alias
of `to_not`.
* doc/specl.md (Inverting a Matcher with not): Note equivalence
of `to_not_` and `not_to`.
* NEWS: Update.

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