Test and fix to vec-final

[izderadicka]

Fixed Iterator implementation for Drain
Added some tests to code

@RalfJung - as discussed in #71 I've created separate PR for these two changes
@steveklabnik - in #71 you noted that test should run during mdbook test, but it does not seem to be so (if I modify a test to fail nothing happens) - is there anything else needed for tests to run?

[izderadicka]

@steveklabnik - I think I get it with tests - mdbook is running doctests only - so I modify code bit to run tests as doctests - also tests are now hidden, as main function is hidden too, so they do not appear in book - to make final example clearer.

[RalfJung]

@izderadicka so you verified that the tests are now run by mdbook test?

[RalfJung]

Maybe insert a different value here so we can distinguish them, making sure the right one gets indeed popped?

[izderadicka]

@RalfJung - Yes test are running now - as doctest is running main and main is running our tests. Also tested that if assertion fails then mdbook test fails as the test panics.

[RalfJung]

LGTM except for changing v.insert(0, 1); to v.insert(0, 2); (or any value other than 1 and 10).

[izderadicka]

@RalfJung - changed test as required