Renames ecparams to curve to avoid confusion

`ECPointFp.decodeFrom` specifies its first parameter as `ecparams`, which is a suitable name on its own; but, it is ambiguous when combined with its usage:

```
var decode = function(x) { return ECPointFp.decodeFrom(ecparams.getCurve(), x) }
```

Therefore, as implied by the above usage and inherent nature of the `decodeFrom` function, I have renamed the first variable to `curve`.

See the `ECPointFp` constructor for another example of why this makes sense.