Allow multiple handlers in the same binary

For other languages on Lambda, a single deployment artifact (JAR or zip file) can have multiple handlers functions in them. Multiple Lambda functions can use the same deployment artifact, with different behavior based on the handler selected in the Lambda function configuration. For Go, this isn't possible. `lambda.Start(handler interface{})` takes a single function. This means a separate binary has to be built for every function, and since Go builds are statically linked, there's no speedup when functions share code. This has caused users create custom build systems [like Fender has](https://medium.com/fender-engineering/building-automating-and-deploying-serverless-microservice-infrastructures-53bd4ca6faf1) to parallelize their builds.

This would be in concert with the Lambda service supporting handler function specification for Go functions in the API.