* Move all public traits to api crate
* Make event handler initialize function a constructor and invert the control over the event handler object creation
* Make Transport trait contain its Response type as an associated type
* Remove type parameters from LambdaRuntime trait and associate it with 2 associated types Handler and Transport
* Remove redundant type parameters from DefaultRuntime
* Adjust the implementation of the DefaultRuntime to follow the new api
* Make EventContext struct non-generic and directly implement all the read and write traits on it
* Rename trait RuntimeEnvVars -> LambdaEnvVars
* Make trait LambdaContext extend LambdaEnvVars
* Create trait LambdaEnvSetter that is responsible for setting any lambda env variables that change between invocations
* Create trait LambdaContextSetter that is responsible for setting any lambda context variables that change between invocations
* Remove redundant arguments from create_runtime and default_runtime macros
* Adjust EventHandler function signatures
* Adjust Docs