move CompletableProcessor and SingleProcessor out of concurrent.api

[Scottmitch]

CompletableProcessor and SingleProcessor both currently live in concurrent.api due to usage patterns and convenience. However these classes are not strictly part of the public API and would be more appropriately placed in an internal package somewhere.

CompletableProcessor currently extends concurrent.api.Completable because it is used in concurrent.api and composed over and/or directly returned as a concurrent.api.Completable. After some concurrent reshuffling (#316, removal of subscribe, clarify concurrent classes from concurrent.api, etc...) we should explore converting this from the "base class" to the api variant as needed to decouple from the concurrent.api package.

[jayv]

Done #440