deprecate <<, >>, >>> on BitArray

[JeffBezanson]

I was surprised to find we still have these methods. Instead, we should have a shift[!] function that works on any array. Using >> to move BitArray elements is arguably a pun, and I think the way we define it is arguably wrong:

julia> BitVector([1,1,1]) >> 1
3-element BitArray{1}:
 0
 1
 1

>> moves elements towards higher indices, while for integers >> moves bits towards lower bit indices.

[StefanKarpinski]

Would obviously be technically breaking. I guess the question is whether anyone is using this. If not, then this could potentially be considered a "minor change".

[mbauman]

I could have sworn that we did this, but looks like I was thinking of #23404. Bummer.

I'm in full support of its deprecation — the only question is when we should do it. We should probably constrain ourselves to only remove the methods in 2.0. If that's the case, is it worth adding a deprecation to 1.2? Or should we wait until we're closer to 2.0?

[JeffBezanson]

There's some discussion of the process here: JuliaLang/Juleps#51. These would become silent deprecations in 1.x, and either be removed or become noisy in 2.0. Unless, I guess, we can really be convinced nobody is using them.