Improve performance of `repeat` expression

What is the problem the feature request solves?

CometStringExpressionBenchmark shows poor performance:

OpenJDK 64-Bit Server VM 11.0.22+7-LTS on Mac OS X 14.6.1
Apple M3 Max
repeat:                                   Best Time(ms)   Avg Time(ms)   Stdev(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------------------------------
Spark                                                10             43          31          0.1        9282.0       1.0X
Comet (Scan)                                         10             84          30          0.1       10044.0       0.9X
Comet (Scan + Exec)                                  31             91          28          0.0       30348.9       0.3X

Describe the potential solution

Review benchmark to make sure it is valid, then try and optimize.

Additional context

No response