Add "new with at least n primes" function to Sieve

[wackywendell]

This adds a function to Sieve to create a new Sieve instance that has at least n primes in it, for a given value of n.

This also adds an implementation of Iterator and IntoIterator for Sieve, just for convenience.

As discussed in #3, the name new_at_least is pretty terrible, but I couldn't come up with anything better. I'm hoping for suggestions!

[autohuonw]

r? @huonw

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[huonw]

Woo, thanks.

I haven't come up with any other names I like yet...

[wackywendell]

Yes, neither have I. The best I've thought of is new_with_count, which does seem a bit more descriptive, but I'm not sure I like it...

[autohuonw]

:umbrella: The latest upstream changes (presumably 6806007) made this pull request unmergeable. Please resolve the merge conflicts.