Implement Built-in Functions for String Operations

[aannleax]

We currently do not support any kind of operations on string values.

It would be desirable to have:

• Filter conditions on existing strings like string-length, contains, starts-with, ends-with, matches
• Creating new string with functions such as concatenate, substring, upper-case, lower-case, replace

A slightly more complete list can be found here: https://www.w3.org/TR/rif-dtb/#Functions_and_Predicates_on_Strings

Implementing functions on string is difficult in the current architecture, since we store them as integers that represent a key into a dictionary. While having a ColumnScan that has read-access to the dictionary seem plausible, having write access into the dictionary for implementing the second kind of operations could be problematic. This is (a) because Rust makes it difficult to work with mutable references and (b) because the new values that were entered into the new dictionary could be discarded later in the computation, thereby wasting space. Here, one could use (and tidy up) the approach for handling nulls, where a snapshot of the current state is fixed before executing the plan. One would then need a method to "commit" changes after finishing the computation.

[aannleax]

Has now been implemented