Add some missing tailrec annotations for clarity

[Blaisorblade]

Clarify these methods are tail-recursive (and ensure they are).

[allanrenucci]

I will quote Martin here:

Add a @tailrec only if it is important that the method is in fact tail-recursive. Either it is super-performance critical or can potentially recurse deeply. Do not add @tailrec just because the method happens to be tail recursive.

Closing. Feel free to reopen if you disagree