Define mechanism to deprecate and remove std lib functions (before we reach 1.0.0)

[dlespiau]

We have no stdlib API guarantees until we reach 1.0.0 but it would still be nice to leave users the opportunity to fix their code for a whole cycle before removing the API entry point. Even better, we could tell them how to fix things.

Proposal:

• Print a warning on stderr for a whole 0.x.0 cycle (or two) with:
  • a way to fix it
  • a link the wiki explaining the rationale
  • the release that is going to remove the API entry point.

It may not always be technically possible to do the above, but we can try.

[dlespiau]

Deprecations are now documented at least: https://github.com/jkcfg/jk/blob/master/docs/deprecations.md