Clean up AttributeUtils

[cmnrd]

While trying to use the Attribute syntax I found that the AttributeUtils could use some cleanups. In particular, this change removes the grammar rule AttrParmValue and instead simply uses Literal. Since some of the type checking done required new functions for checking literals, I also decided to move all functions regarding expressions to a new class ExpressionUtils. And while doing so I also realized that AstUtils, ExpressionUtils, and AttributeUtils woulb probably better located in the org.lflang.ast subpackage. I realize that this last change might be causing conflicts with other large outstanding PRs. Feel free to revert the last commit if it seems too intrusive.

[oowekyala]

I understand where this is coming from, but maybe we should wait to move these classes. The AST nodes themselves don't live in the ast package -maybe they should be moved there as well. I'm partly saying this because #1441 is still in flight as you know, and I want it to avoid the fate of #544.

I suggest we should have a ticket about planned package restructurings, and do everything in bulk as a planned effort when that achieves a critical mass.

[cmnrd]

OK, we could add it as an item to #1276.

It looks like the tests do not pass anyway, I will probably only get to fix them end of next week.

[lhstrh]

This is cool, but I'd prefer to defer merging this into master until fed-gen is merged. We're getting pummeled with conflicts otherwise, making it even harder to get it ready to merge.

[cmnrd]

I force pushed a more minimalist version today, that only cleans up the attributes but avoids additional refactoring.