Add additional implementations.

Update README.md

At some point we discussed whether APIs where the logger is optional should use
"Logger" and document that it might be the null logger or use "*Logger". We
settled on the later, but don't seem to have that written down.

docs: describe handling of optional logger parameters

Also fix a lint-warning, which ensures the tests have something to test.

Fix lint error and tests for Go versions

Produce a maybe-useful error rather than panic when we are in the middle
of logging a value (similar to fmt.Printf).

funcr: Handle nil Stringer, Marshaler, error

This avoids the pkg name overlap with standard "testing".

Move testing -> testr, deprecate, alias old names

Format and add tests