MONGOID-5662 Deprecate Object#__to_inc__ and BigDecimal#__to_inc__

[jamis]

This extends @johnnyshields work in #5709 and deprecates Object#__to_inc__ and BigDecimal#__to_inc__.

Though these are implicitly private API methods, they were never explicitly declared as such, and so we're opting to deprecate them for now and remove them in a later major release.

[johnnyshields]

Jamis, thanks for actioning these.

I'm happy to spend some time on the weekend updating my other PRs to preserve existing methods and deprecate them if it would help. Ah it looks like you're just doing this for the cases where it wasn't explicitly declared private or @api private...

[jamis]

@johnnyshields -- right. Dmitry and I discussed it and agree that unless a method was explicitly declared private or @api private, it must be implicitly public, and thus must be deprecated before being removed. It's annoying red tape, but we're preferring to be cautious.

MONGOID-5660 is my focus this week, and I'm optimistic about getting through them all by the end of the week. The groundwork you've laid for these is greatly appreciated!