Remove all uses of `todo!`

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

We have a number of uses of todo! that we should replace with use of Err.

$ find . -name "*.rs" -exec grep -H "todo\!" {} \;
./src/expr/indexed_field.rs:            _ => todo!(),
./src/expr.rs:                ScalarValue::Null => todo!(),
./src/expr.rs:                ScalarValue::Decimal256(_, _, _) => todo!(),
./src/expr.rs:                ScalarValue::FixedSizeBinary(_, _) => todo!(),
./src/expr.rs:                ScalarValue::List(_) => todo!(),
./src/expr.rs:                ScalarValue::Struct(_, _) => todo!(),
./src/expr.rs:                ScalarValue::Dictionary(_, _) => todo!(),
./src/expr.rs:                ScalarValue::FixedSizeList(_) => todo!(),
./src/expr.rs:                ScalarValue::LargeList(_) => todo!(),
./src/expr.rs:                Operator::AtArrow | Operator::ArrowAt => todo!(),
./src/common/data_type.rs:                todo!()
./src/common/data_type.rs:            ScalarValue::LargeList(_) => todo!(),

Describe the solution you'd like

Describe alternatives you've considered

Additional context

[drauschenbach]

There appears to be just one todo!() left: indexed_field.rs. #good_first_issue