`->` has a scoping bug

Needs investigation. The following two pieces of code should be equivalent but aren’t:

```r
lambda = function (x) {
mod::import('klmr/functional/lambda', '<-')
lapply(seq_along(x), i -> x[i])
}
```

```r
lambda2 = function (x) {
mod::import('klmr/functional/lambda', '<-') # For consistency with the above
lapply(seq_along(x), function (i) x[i])
}
```

Called as:

```r
lambda(1 : 5)
lambda(1 : 5)
```

The second implementation works as expected; the first works for the first time, but fails when called again with:

> Error in match.fun(FUN) : object 'i' not found

To make matters worse, the error vanishes when the function is being debugged. It *also* vanished if the `mod::import` call is pulled out of the function into the global namespace.