Move hash table code from execution package to util package

The code for hash table is currently in the execution package and thus can only be used by code that's either in the execution package or can import the execution package. This limit its usage. For example, we want to have IN-list expression to be able to use the hash table code for evaluation of the IN-list, and the expression package cannot import the execution package.

By moving the hash table code to a more generic package such as the util package, it can be used in many other places.