* codeFixInferFromUsage: Assume that using `x[0]` means that `x` is an array
* Remove unnecessary '||' with non-falsy LHS
If only there were some kind of type-checker for JavaScript that could detect this sort of thing
* Fix inferFromUsage on index signatures
1. Check the argumentExpression to determine the index signature type.
Previously, the code mistakenly checked the type of the element access
itself, which never returned a good type.
2. If inference for the index signature type fails, substitute anyType.
Previously, the code would create a bad index signature with an
undefined type.
3. Add tests. Previously, there were no tests.
* Fixing (1) means that number index signatures are now created.
Previously, only string index signatures would be created.
* Fixing (2) means that index signatures will be inferred from single
usage like `return a[0]`. Previously, the refactoring would fail,
perhaps because of a check when stringifying the index signature (I
haven't tracked down why.)
* Update fourslash test with improved inference
