autoCurry refactor to remove unused else branch

This is not juicy, but the autoCurry function had code that was not reachable. This change simply cleans it up:
```
if arguments.length < numArgs
then numArgs - arguments.length is always > 0
```
All the tests pass and I added 2 sanity tests (justifies the need for `argdiff > 0` instead of just `argdiff`), but you may decide that calling a curried function with too many arguments should throw an error; I could argue both ways.