[Feature] Add handlers registry

[bsless]

Just like middleware registry, but with a few open questions:

• Should it be an opt of ring.core/router?
• How should resolution be performed? Just postwalk and look for :handler?
  • Does that risk mangling input data?
  • Same for metadata (meta merge)
  • Can be done after merge but before compilation
• Belong in core or ring?

[ikitommi]

Quick comments:

• base into reitit.middleware which defines concept of :handler, ring uses it
• :handler paths are known in the :compile hook, different for middleware & ring, this is the way
• maybe IntoHandler protocol?