append_nulls and append_trusted_len_iter for PrimitiveBuilder

[bjchambers]

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

I have cases where I've identified a range of numbers to add to a builder. Or a number of nulls to add. My only options with the existing PrimitiveBuilder interfaces is to either (a) add each value/null individually or (b) create a vector containing the numbers/nulls I wish to add, and then use append_values or append_slice.

Both of these extra work than is strictly necessary -- in one case, capacity checks after every value and in the other materializing an intermediate vector.

Describe the solution you'd like

• pub fn append_nulls(&mut self, num_nulls: usize) -> Result<(), ArrowError>. Same as calling append_nulls num_nulls times.
• pub fn append_trusted_len_iter_values(&mut self, iter: impl IntoIterator<T::Native>) -> Result<(), ArrowError>. Behaves similarly to collecting the items into a vector and then calling append_slice(&vector).
• Could also add a version for Option<T::Native>.

Describe alternatives you've considered
As noted in the description -- multiple calls to append_value/append_null (not ideal due to checking the capacity after each call) or creating a vector and then calling append_values or append_slice (not ideal due to the need to collect/materialize values first).

[bjchambers]

I have a prototype of these methods, that improved the performance of a 2-way merge kernel 20-30% in some 100k element microbenchmarks.

https://github.com/bjchambers/arrow-rs/tree/725-append_nulls_iter