* add method to fetch block of first vals in columnar
add method to fetch block of first vals in columnar (this is way faster
than single calls for full columns)
add benchmark
fix import warnings
```
test bench_get_block_first_on_full_column ... bench: 56 ns/iter (+/- 26)
test bench_get_block_first_on_full_column_single_calls ... bench: 311 ns/iter (+/- 6)
test bench_get_block_first_on_multi_column ... bench: 378 ns/iter (+/- 15)
test bench_get_block_first_on_multi_column_single_calls ... bench: 546 ns/iter (+/- 13)
test bench_get_block_first_on_optional_column ... bench: 291 ns/iter (+/- 6)
test bench_get_block_first_on_optional_column_single_calls ... bench: 362 ns/iter (+/- 8)
```
* use remainder
