os.path equivalent for fsspec

One of the things that have come up when trying to integrate fsspec into tensorboard (https://github.com/tensorflow/tensorboard/pull/5248) is that there aren't any standard path operations as part of fsspec as far as I can tell. When dealing with more complex chained filesystems the rules get pretty complex for users to implement.

Ideally fsspec would provide the common operations such as:

• join
• basename
• dirname

I.e.

>>> path = "simplecache::zip://foo::s3://bucket/path.zip"
>>> fsspec.path.join(path, "bar") 
"simplecache::zip://foo/bar::s3://bucket/path.zip"
>>> fsspec.path.dirname(path)
"foo"
>>> fsspec.path.basename(path)
"simplecache::zip://::s3://bucket/path.zip"

With the chained filesystems this gets really complex and I'm not 100% sure how to implement this in all cases/filesystems so feedback would be appreciated here

An option here might be to try and implement https://docs.python.org/3/library/pathlib.html#pathlib.PurePath

No file or test is named in the issue. Start by examining how fsspec currently parses and handles chained filesystem paths, then compare the desired join, dirname, and basename behavior with pathlib.PurePath. Done means providing consistent operations for the illustrated chained-path cases and clarifying behavior across filesystem combinations.