split func dialect into smaller dialects

[Roger-luo]

I'm going to halt for splitting the func dialect for now - splitting call/invoke into an independent dialect is simple, splitting lambda out of func is not - it's part of the function syntax in Python so you cannot really do the split, but instead creating two dialects (one dialect allows nested function, one dialect doesn't).

I don't think we have a use case to restrict nested functions and function calls except for iQuHack at the moment. So let's just assert that in the validation for iQuHack analysis.

If someone has a new use case, feel free to contribute a new dialect that restricts nested functions with a different lowering rule.

[Roger-luo]

ok based on recent split of cf lowering strategies into a new lowering.cf dialect, there is a new way of doing this.

1. we split func dialect into func, call and closure dialect (well lambda is a keyword)
2. we move lowering into separate dialects lowering.func does not allow nested functions (thus no closure), lowering.closure allow both normal and nested functions and maybe lambda?
3. so then you need to combine func, call closure, lowering.closure, lowering.call to recover the current func equivalent.