schema annotation should add cachedImplicit

[durban]

The @schema annotation should also add an implicit definition with shapeless.cachedImplicit (or equivalent) to the companion object (thus optimizing compile time for the uses of the Reified instance).

Problems:

• [x] If the companion object already contains implicit definition(s), they could conflict with the added one. [Solution: this should be detected, and in these cases, the automatic one shouldn't be added.]
• [x] We can't detect already existing conflicting implicits (because it seems that we cannot typecheck in an annotation macro). [Solution: add the implicit unconditionally and document it.]
• [x] The implicit defined in the companion object will conflict with the ones in the Reified companion object. [Solution: fb5b1131c34c18ffcb157e6949793bf780b6d0df]

Tasks:

• [x] Add implicit definition to the macro.
• [x] Check if we still need the forwarder. [57aae75]

[durban]

TODO:

• [x] take a look at Exported[_] in circe
• [x] take a look at LowPriority and Orphan in shapeless