simplified hash calculation for varchars in constant time

[markonussbaum]

A very simple, but fast hash implementation for nullable_varchar.

Instead of iterating over all characters in O(n), it just samples the first, last and median element of the varchar in O(1), resulting in a speedup in many cases.

Result with TPC-H at scale 10 on vh001:

Old implementation:
real 64m30.605s
user 0m15.766s
sys 0m2.302s

With this PR:
real 49m39.123s
user 0m14.275s
sys 0m2.093s

The test was done with

• aggregate-on-ve
• amplify-batches
• --num-executors=1 --executor-cores=8