x[i] notation

Suggest that the notation x[i] and x[[i]] be allowed in mp in which case these would all be the same

```
p1 <- mp("x1 * x2 + x1", stars = TRUE)
p2 <- mp("x[1] * x[2] + x[1]", stars = TRUE)
p3 <- mp("x[[1]] * x[[2]] + x[[1]]", stars = TRUE)
```

Also `as.function(mp("x[2] + x[1]", stars = TRUE))` would use x[1], x[2] as the passed vector and not x[2], x[1].

Some method of specifying a sorted varorder in as.function would be nice as opposed to having to do this.

```
p <- mp("2*x2 + x1", stars = TRUE)
as.function(p, varorder = sort(vars(p)))
```