This brings things more into line with modern conventions for
build target names, and means we have fewer sources of truth for
the minimum Go version that's needed.
* 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.
