## Adds `/eval` command

`/eval` takes lua code as argument and executes it. It will echo your command and show it's output and returned value.
Each player gets their own "global" environment so they can't interfere with other user's envs by accident (exposed as `cmd_eval.e[player_name]`).

## Some nice features:
### Expression/statement agnostic
This command:
```
/eval 1+2
```
Outputs this:
```
> 1+2
| 3
```

This also just works:
```
> x = 2*2

> x
| 4
```

Multiple values also work:

```
> return 1,nil,3
| 1,
| nil,
| 3
```

### Backtrace and error output
Outputs both the error and clean backtrace (stack related only to provided code)

### Print function
No need to use `core.chat_send_player()`, just use `print()` - it will do the right thing.

```
> print(here)
< (-98.0, 15.5, 33.4)
```

```
> objs = core.get_objects_inside_radius(here, 10)

> for i,o in ipairs(objs) do print(i, (o:get_luaentity() or {}).name, o:get_pos()) end
< 1 nil (-98.0, 15.5, 33.4)
< 2 mobs_animal:kitten (-101.0, 16.5, 30.0)
< 3 mobs_animal:chicken (-92.0, 15.5, 34.2)
```

### "Magic" variables
Some special variables are provided:
- `here` - position where you executed the command
- `me` - your player object
- `point` - point in the world you're pointing at with your crosshair
- `this_obj` - entity you're pointing at (can be `nil`)

Do `/eval help` to get a quick reminder of these vars and functions.

### Better output for arrays and some `userdata` objects
```
> me
| #<player: "singleplayer">
```

Show indices for easier manual access:
```
> core.get_objects_inside_radius(here, 10)
| {
|  [1] = #<player: "singleplayer">,
|  [2] = #<luaentity: "mobs_animal:kitten">,
|  [3] = #<luaentity: "mobs_animal:chicken">,
| }
```

### Keeping result of last /eval in `_` variable
```
> core.get_objects_inside_radius(here, 10)
| {
|  [1] = #<player: "singleplayer">,
|  [2] = #<luaentity: "mobs_animal:kitten">,
| }

> _[2]
| #<luaentity: "mobs_animal:kitten">

> pos = _:get_pos()

> pos
| {
|  x = 123.5,
|  y = 15.0,
|  z = 68.4
| }
```

### Accessing and changing global variables
You can save some keypresses by not typing `local` in front of every
variable. Assigning to globals won't clobber them, instead using your personal
environment. But you can still read globals as usual.

```
> print(myvar)
* Accessing undeclared variable: "myvar"
< nil

> myvar = 1
| Done.

> print(myvar)
< 1
| Done.
```

If you want to create a new global or actually overwrite existing one,
you can access global environment through usual `_G` or `global`
variables. Using them will print a message telling you if global var
already existed.

```
> cmd_eval = nil
| Done.

> cmd_eval  -- accesses real global
{
  e = { ... }, ...
}

> global.cmd_eval = nil
* Overwriting global: "cmd_eval"
| Done.

> cmd_eval
| nil       -- wiped for real
```

### Other shortcuts

#### `dir()` and `keys()`

List keys of the table (useful for exploring data structures, without flooding your chat).

#### `get_objects_inside_radius()` shortcuts: `goir()` and `oir()`

`goir(radius)` returns a list of objects around you
`oir(radius)` returns an iterator of objects around you

```
> goir(100)   -- return a list of objects within 100 units around you

> for v in oir(100) do print((v:get_luaentity() or {}).name) end
```