motivation: define stable API in preperation 1.0 release
changes:
* require swift 5.7, remove redundant backwards compatibility code
* make LambdaHandler, EventLoopLambdaHandler, and ByteBufferLambdaHandler disjointed protocols to reduce API surface area
* create coding wrappers for LambdaHandler and EventLoopLambdaHandler to provide bridge to ByteBufferLambdaHandler
* reuse output ByteBuffer to reduce allocations
* add new SimpleLambdaHandler with no-op initializer for simple lambda use cases
* update callsites and tests
* update examples
Co-authored-by: Yim Lee <yim_lee@apple.com>
Co-authored-by: Fabian Fett <fabianfett@apple.com>