Implement CQL Substring operator

[suyashkumar]

This implements the CQL Substring operator.

One interesting note: what should Substring('', 0) produce? The current code returns an empty string instead of Null (it seems reasonable to get the identical string when doing Substring(str, 0) == str to me). The spec would probably imply Null. Could be worth clarifying/discussing, happy to change that back.

Some code generated with the help of https://github.com/apps/google-labs-jules !