Rename lambda.Lambda to lambda.Function

[eladb]

Since we now encourage using namespaced imports import lambda = require('@aws-cdk/aws-lambda'), it should be fine (at least from JavaScript's perspective) to use Function as the name of the class. If this proves problematic in other languages, jsii generators should mangle the name (similarly to other reserved words).

Also, clean up the class names in @aws-cdk/aws-lambda not to include a Lambda prefix