Deprecate ObjectMapper.canDeserialize()/ObjectMapper.canSerialize() in 2.18

[cowtowncoder]

(note: offshoot of discussion in #4568)

As per #1917 there is limited value in these 2 methods (and underlying machinery), and hence they are dropped from 3.0, as we cannot drop them from 2.x due to backwards-compatibility requirements.
But since the fundamental problem exists, we should mark them as @deprecated in 2.x (specifically, 2.18).

[JooHyukKim]

Along with deprecating the two methods, shall we come up with alternative way to achieve the same functionality which may be added to deprecated methos' javadocs so users know what to do later on?

[cowtowncoder]

@JooHyukKim There isn't really matching functionality, unfortunately: there is no good way (that I know of) to figure it in useful manner whether something is reliably (de)serializable; from what kind of input and so on. Calling applications should not -- for example -- try to obtain serializer or deserializer.

I am +1 on improvements to Javadocs, just not sure what to say other than "these seemed like a good idea but weren't".