Improve test coverage

[istathar]

The parser.rs module was becoming enormous. Move the test code that was in a module in that file to a new location checks/ but jump through some minor hoops so that name didn't need to match the parent.

Also adds a fix to resolve mishandling of blank lines, and adds a new error case if parenthesis are omitted when binding to more than one variable.

Handle the $* special case that resets the attribute assignment.

Prevent the formatter from wrapping code inlines and within string interpolations. This is corner case territory, but it looks rubbish to have a single '}' sitting by itself on a line just because it got wrapped. So, when formatting, we instead keep the binding and surrounding code inline together as an atomic unit.

[istathar]

I'm not convinced that this is the right approach, but it seems the only way to allow test code to see the internals of the module being checked.