Introduce new compiler directive #impure and mark all native components in stdlib with it. Analyzer must restrict usage of this directive to stdlib and native components only - normal components and components outside of stdlib must never use it.

// http

#impure
def Get(...) (...)

This way it would be possible to tell at compile-time that some node-tree is pure or impure. E.g. it would be possible to implement other features like restrict Map to be used with pure components only, so it's always possible to run it concurrently (see #754 for a related problem)

Questions

1. How to handle components with interfaces?
2. What to do with components that does go-interopability (when it will be here)?

In the future when there will be Go interop, component that does it must either be always threaten like "impure" or it's up to developer to mark it (analyzer should allow it then). Maybe there are other options to think about though.

Related to #228