* Extract some test utilities to a separate package.
* Rename some test utilities.
* Internally expose DirectChildren.
* Add LocationRange to some non-expr AST parts,
such as local binds, parameters and object fields.
* Add end-of-file-reached testcases.
There is no reason for external users to directly depend
on parser. It had a few random things exported as well,
namely errors and "children" functions (helpers for AST
traversal).
It was easy to extract the errors package, but I needed to leave
children in parser for now. The errors package was also
made internal, but it's a candidate for making it public
again potentially (if someone wants to display error messages
just like us). For now it's probably too incomplete anyway.
This change has a potential of breaking the existing users
since we technically remove public APIs. These were not needed
or even helpful for actually running Jsonnet code, but perhaps
someone used them anyway.
