Clarifying comments for RETURN_NOT_ALLOWED
No "unreachable code" errors when an illegal return expression is the last in a lambda
Labels on function literal arguments are processed properly
Minor: code cleanup
Duplicating parameters removed
Support local returns in lambdas
GenerateTests uses headless mode
Minor. Local variable renamed
Local returns are only allowed with explicitly specified return types

 This is a temporary limitation: otherwise type inference is having trouble to account for all the returned expressions.
 We will fix the inference and remove the limitation
JVM backend test for local returns
Test that local return has type Nothing
No need to produce error types when RETURN_NOT_ALLOWED_EXPLICIT_RETURN_TYPE_REQUIRED is reported