impl a From instead of an Into

[mightyiam]

itertools/src/either_or_both.rs

Lines 503 to 511 in 5a5731c

	impl<A, B> Into<Option<Either<A, B>>> for EitherOrBoth<A, B> {
	fn into(self) -> Option<Either<A, B>> {
	match self {
	EitherOrBoth::Left(l) => Some(Either::Left(l)),
	EitherOrBoth::Right(r) => Some(Either::Right(r)),
	_ => None,
	}
	}
	}

[Philippe-Cholet]

It originated in #740 (From should be prefered to Into implementation as the first gives the second but it's not true the other way) where I said #740 (comment)

I don't see a point of not doing it so I suggest you convert it to a From implementation in a separate commit.

EDIT: After searching, it originated in #327 and nobody said it should not be From instead.

In #327, there is

A fallible method of converting EitherOrBoth into an Either would be very useful providing exclusive (not including Both variant) methods. Converting should probably wait for rust-lang/rust#33417.

So I guess the question is: Instead of Into<Option<_>>, do we want a TryFrom or a From implementation?
I would vote for From (better than Into, no breaking change) over TryFrom (breaking change, what kind of error?).