swarm/api: FUSE read-write support (#13872)

- Moved fuse related code in a new package, swarm/fuse
- Added write support
  - Create new files
  - Delete existing files
  - Append to files (with limitations)
- More test coverage

swarm/fuse/fuse_root.go

@@ -0,0 +1,35 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// +build linux darwin freebsd
+
+package fuse
+
+import (
+	"bazil.org/fuse/fs"
+)
+
+var (
+	_ fs.Node = (*SwarmDir)(nil)
+)
+
+type SwarmRoot struct {
+	root *SwarmDir
+}
+
+func (filesystem *SwarmRoot) Root() (fs.Node, error) {
+	return filesystem.root, nil
+}