interfaces: introduce and use a protocol class for the `charencoding` module See f2832de2a46c for details when this was done for the `bdiff` module. This lets us dump the hack where the `pure` implementation was imported during the type checking phase to provide signatures for the module methods it provides. Now the protocol classes are starting to shine, because these methods are provided by `pure.charencoding` and `cext.parsers`, and references to `cffi.charencoding` and `cext.charencoding` are forwarded to them as appropriate by the `policy` module. But none of that matters, as long as the module returned provides the listed methods. The interface was copy/pasted from the `pure` module, but `jsonescapeu8fallback` is omitted because it is accessed from the `pure` module directly when the escaping fails in the primary module's `jsonescapeu8()`.