inch IO::Path closer to the recent spec changes (path, volume, Str)

[labster]

Start to support .volume (though eventually we'll need File::Spec for that)

.Str now returns full path, .path now returns self. IO::FileTestable changed to work with that.

[moritz]

I fear that the ".Str now returns full path, .path now returns" part will break File::Tools (or whatever it's called), and thus panda. Have you check that?
If yes, we need an intermediate step: a method like .fullpath which returns the full path, and which those modules can use now, and a deprecation notice in file docs/deprecations. Then we can update those modules in due time, and change the .Str and .path methods after the next Rakudo release.

[labster]

Panda runs locally and passes all of its tests, and so do File::Find and Shell::Command.

File::Find uses .basename, and seems to be designed to work around limitations in IO::Path in the first place.

[labster]

I'd also like to note that a path object stringifies to what .path used to be, so the new .path should not be too surprising. This is why roast for io-path still passes -- though I will need to fix the tests for dir(). And add lots more tests, while I'm at it.

.Str changing is more likely to be a surprise, but the spec change was announced more than 2 weeks ago, which I hope is long enough.