cleanups general code and public API

[olme04]

• revert ensures Payload released in LimitingFlowCollector #172
• remove unused data packet from ErrorFrame
• change Frame.type from field to getter
• expose FrameType to public
• cleanup usages of DangerousInternalIoApi on ChunkBuffer - after ktor 1.6.0 it's no more internal
• move some classes to internal package
• change visibility from public to internal for some functions and deduplicate several functions (like SafeChannel) and use hack with invisible reference to use internal declarations in other modules - TODO may be find some better way to share such internal declarations between core and TCP/WS/Local transports implementations
• use sealed interfaces for mime-types and payload builders
• use fun interfaces in places with suspend fun
• make logging api explicitly public and provide temporary experimental annotation for it
• enable explicitApi mode