Change current working directory with Lua

Need something like `fs.chdir` to change current working directory with Lua.

Example usage:
```
local err = fs.chdir("/this/path/exists")
assert(err == nil, "Failed to chdir to existing directory")

local err = fs.chdir("/no/such/path")
assert(err != nil, "Chdir to nonexistent directory succeeded")
```

`err` should be string describing the failed syscall.