an in-memory Lua file system

ch1. minimal directory structure

ch2. think about file, hardlink and node

ch3. dir and getnode

ch4. mkdir : add a directory into another

ch5. special rootdir

ch6. paths

ch7. chroot and mount point

ch1. minimal directory structure

one directory
```
d = {}
d["."] = d
```

2 directories

a parent directory
```
parent = {}
parent["."] = parent
```
a child directory
```
child = {}
child["."] = child
```
link them together
```
parent["CHILD"]
child[".."]=parent
```

object VS index

```
d = {}
d.tree = {}
d.tree["."] = d
```

before : d["CHILD"]
after  : d.tree["CHILD"]

foo/bar/baz

before : d1["foo"]["bar"]["baz"]
after  : d1.tree["foo"].tree["bar"].tree["baz"]

ch2. think about file, hardlink and node

file != directory, but ...
common part : (mainly) hardlink
generic name for common part ? a node
node
node -> dir
node -> file

node and hardlink

local node = class("node")
function node:init()
        self.hardcount = 1
end
-- create a hardlink of (dir|file)<what> named <name> into (dir)<self>
function node:hardlink(name, what) [...] end
function node:unhardlink(name) [...] end

ch3. dir and getnode

local dir = class("dir", nil, node_class)
function dir:init(parentdir)
    self.tree = {}
    self:hardlink(".", self)
    if parentdir then
            self:hardlink("..", parentdir)
    end
end

function node:getnode(t)
    return cur.tree[name]
end
function node:__div(t) [...] end

sample

```
child = parent.tree["CHILD"]
```
```
child = parent:getnode("CHILD")
```
```
child = parent / "CHILD"
```
foo/bar/baz
```
baz = foo.tree["bar"].tree["baz"]
```
```
baz = foo:getnode("bar"):getnode("baz")
```
```
baz = foo / "bar" / "baz"
```

ch4. mkdir : add a directory into another

local dir = require "memfs.dir"
 
local d = dir(nil)
local dd = dir(d)
d.tree["CHILD"]=dd
 
dd.tree[".."] == d
d.tree["CHILD"] == dd

mkdir, rmdir

mkdir

function dir:mkdir(name)
    if self.tree[name] then
            error("already exists", 2)
    end
    local d = instance(dir,self)       -- new directory d
    self.tree[name] = d                -- add d
    return d
end

rmdir

function dir:rmdir(name)
    if not self.tree[name] then
            error("not exists", 2)
    end
    self.tree[name]:unhardlink("..")
--      self.tree[name]:unhardlink(".") -- hmm no
    self.tree[name] = nil
    return nil
end

To be continue ...