Prevent fn.curry from currying a function more than once

This makes `fn.curry` check to see its argument has already been curried. If it has, there is no need to curry it again. This improves performance, because if you curry a function more than once, it has to call through a stack of functions to get to the real one. In my testing, after currying a function 5000 times the original way it took 24ms  to execute the function. After currying it 5000 times in the new way it only took 1ms to execute. Also, running a function that was curried 10,000 in the old way caused a crash, whereas in the new way 10,000 curries is the same as 1 curry.

You can imagine the situation where a function takes an operator, for example `eq`, and wants to curry it.
```javascript
function find(eq, collection) {
    eq = fn.curry(eq);
    ...
}
```

It's nice not to have to worry about whether or not `eq` was already curried before it was passed to find.
```javascript
var eq = fn.curry(fn.op['===']);
find(eq(1), collection);
```