Codegen for string interpolation needs to use wrap the concatenation in parentheses

This

```moonscript
"a" / "#{b}.c"
```

compiles to:

```lua
"a" / tostring(b) .. ".c"
```

which is evaluated as:

```lua
("a" / tostring(b)) .. ".c"
```

This is different precedence than one would think at first glance. It should be:

```lua
"a" / (tostring(b) .. ".c")
```