MutableString: fix an off-by-one preventing final character results.

* lib/zile/specs/MutableString_spec.yaml (chr, find, rchr, rfind):
Add specifications for returning last checked position when
starting seach from the last checked position.
* lib/zile/MutableString.lua (find): Fix an off-by-one error to
meet new specifications.

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