Improved performance of utf8 check for ascii-only (-40% parquet reading ascii-only columns)

[jorgecarleitao]

read utf8 2^20          time:   [8.1475 ms 8.2333 ms 8.3429 ms]                           
                        change: [-42.546% -41.204% -39.859%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)

Big kudos to @ritchie46 for point out this optimization on pola-rs/polars#1553

There are other optimizations on that PR, but this is already pretty big for a +5 ^_^

[jorgecarleitao]

Closing in favor of #542 ; (man, that was pretty impressive synchronization :D)