Problem: having public as the default makes it very easy to accidentally
export local function by forgetting the `private` modifier.
Solution: functions are private by default and must be declared as `entrypoint`s
to be exported. So `entrypoint foo() = ...` instead of `function foo() = ...`.
We still accept the `private` modifier although it is redundant.
Add an include directive to include namespaces into a contract. Only allowed at the top level.
To allow includes, either call through aeso_compiler:file or set the option `allow_include` (and add `include_path`(s)).
