`FSMap.setitems` fails when `FSMap.__setitem__` succeeds (nested directory on local filesystem).

When using a mapper on a local filesystem, it's possible to create nested subdirectory structures using FSMap as in:

import fsspec
import tempfile

with tempfile.TemporaryDirectory() as d:
    m = fsspec.get_mapper(d)
    m["foo/bar"] = b"baz"

however, trying the same thing using setitems fails:

import fsspec
import tempfile

with tempfile.TemporaryDirectory() as d:
    m = fsspec.get_mapper(d)
    m.setitems({"foo/bar": b"baz"})

I'd expect the latter to work just as well as the former, however, it raises:

FileNotFoundError: [Errno 2] No such file or directory: '/tmp/tmp4msurdfc/foo/bar'

This is likely because there's no mkdirs in setitems, while there is one in __setitem__. I don't know which place would be the best to put a mkdirs-like call when setting multiple items at once.

Start in fsspec/mapping.py by comparing setitems with setitem, especially their directory-creation behavior. Reproduce the nested-key examples from the issue, then add regression coverage and confirm that setitems handles keys such as "foo/bar" on a local filesystem without raising FileNotFoundError.