Speed up the substring kernel

[HaoYang670]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
https://github.com/apache/arrow-rs/blob/master/arrow/src/compute/kernels/substring.rs#L44

    let mut new_values = MutableBuffer::new(0); // we have no way to estimate how much this will be.

We can estimate the memory we need by firstly calculating the length of each substring. This can improve the performance because we don't need to reallocate memory when building buffer.