specs: take advantage of improvements to Specl DSL.

* specs/container_spec.yaml, specs/debug_spec.yaml,
specs/functional_spec.yaml, specs/io_spec.yaml,
specs/list_spec.yaml, specs/math_spec.yaml,
specs/object_spec.yaml, specs/optparse_spec.yaml,
specs/package_spec.yaml, specs/set_spec.yaml,
specs/std_spec.yaml, specs/strbuf_spec.yaml,
specs/string_spec.yaml, specs/table_spec.yaml,
specs/tree_spec.yaml: Use `not_to_` instead of `should_not`,
`to_` instead of `should_` and `to_copy` instead of
`should_equal` plus `should_not_be`.

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