refactor: make all matcher methods OO.

* lib/specl/matchers.lua (Matcher.matchp, Matcher.format_expect)
(Matcher.format_actual, Matcher.format_alternatives): Add self
parameter. Adjust all callers.
* lib/specl/shell.lua, specs/spec_helper.lua: Likewise.
* doc/specl.md (Custom Matchers): Update documentation.
* NEWS: Note incompatible changes.

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