Merge #829 #830

829: Constant folder now returns error_mark_node instead of nullptr r=philberty a=npate012

Removed nullptr checking on results from constant folder because when the
result is already error_mark_node, we no longer need to check if the result
is nullptr.

Fixes #692

Signed-off-by: Nirmal Patel <npate012@gmail.com>


830: Cleanup lambdas within some AST types r=philberty a=dafaust

Cleanup constructs like:
```C
    struct_decl.iterate ([&] (AST::TupleField &field) mutable -> bool {
        ... /* do stuff with field */
    });
```
For the following AST classes:
- AST::StructStruct
- AST::TupleStruct
- AST::Union
- AST::ArrayElemsValues

Fixes: #714
Fixes: #715
Fixes: #716
Fixes: #720


Co-authored-by: Nirmal Patel <npate012@gmail.com>
Co-authored-by: David Faust <david.faust@oracle.com>