expose a preconfigured dirfs instance on mapper objects

[keewis]

• closes using filesystem methods with mapper keys #1269

I'm pretty happy with the way dirfs instances help working with mapper objects, but constructing one that uses the same parameters as the mapper is a bit more work than I would wish:

m = fsspec.get_mapper(url, ...)
dirfs = fsspec.filesystem("dirfs", path=url, fs=m.fs)
dirfs.open(...)

As suggested in #1269, this exposes a dirfs instance as a cached property, which means that anyone who doesn't need the dirfs instance doesn't have to wait the (very) little time it takes to construct one. With that, the above snipped would become:

m = fsspec.get_mapper(url, ...)
m.dirfs.open(...)

I'm not particularly attached to the name of the property, so I'm happy to change that.

Also, I didn't discuss this change before implementing it, so feel free to close if you don't think growing the mapper API makes sense in this case.

[martindurant]

I am happy with this. I only request a short docstring on the attribute explaining what it does, and to include it in the API doc for FSMap.