Redundant tests for the first/last operators

[cartant]

Whilst fixing the typing problems in the test suite, I found a seemingly redundant test for the first operator:

it('should support type guards without breaking previous behavior', () => {

first does not accept a used-defined type guard, so the test does nothing. Indeed, it will fail once type checking is enabled for the test suite.

Whether or not the test should be removed or the signature of first should be modified to accept a user-defined type guard, I don't know. Hence the opening of this issue.

[cartant]

There is a similar problem with last.

[benlesh]

Yeah, it just looks poorly written. We do need to make sure that type guards work with operators like first, last, filter, etc.

[cartant]

I'll come back and fix these up, then, once the test suite is type-checked. Some overload signatures will need to be added for the type guards to effect narrowed observable types.