Add entries to MapBuilder to return both key and value array builders

[viirya]

Which issue does this PR close?

Closes #.

Rationale for this change

MapBuilder provides keys and values functions to get key and value array builders of map separately. As it is mutable borrow of the map builder, when we want to set keys and values in a loop like:

let key_builder = builder.keys();
let value_builder = builder.values();

let keys = ...;
let values = ...;

for idx in 0..keys.get_num_elements() {
    let key = keys.get(idx);
    let value = values.get(idx);

    key_builder.append_value(key);
    value_builder.append_value(value);
}

The compiler will complain error[E0499]: cannot borrow *builder as mutable more than once at a time.

Although there is workaround that is to get key and value builders inside the loop, no sure if it is efficient. More, especially we can have Box<dyn ArrayBuilder> as key and value builders now, it means that we need to downcast both key and value builders for each iteration in the loop.

This patch adds entries API to MapBuilder that can get both key and value builders in one mutable borrow.

What changes are included in this PR?

Are there any user-facing changes?

[tustvold]

Suggested change

	pub fn keys_and_values(&mut self) -> (&mut K, &mut V) {
	pub fn entries(&mut self) -> (&mut K, &mut V) {

Perhaps?

[viirya]

Looks good to me.

[viirya]

Thank you @tustvold