Improve readability and maybe performance of string --> numeric/time/date/timetamp cast kernels

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
While examining the code, I found that we could make the string --> numeric compute kernels faster buy using Rust iterators to avoid bounds check and avoiding string allocations for the "safe cast" paths

Describe the solution you'd like
faster kernels

Describe alternatives you've considered

Additional context