Add support for substr() function

[tysonzero]

Currently it does not seem possible to slice up strings.

[charukiewicz]

It's pretty easy to add support for new functions in a similar manner to some of the existing (like length_, lower_, upper_, rtrim_, etc.). Here's how and where they're implemented:

https://github.com/bitemyapp/esqueleto/blob/a61f5527e8d8c2d4ce679b295578f9fe4dafd7cf/src/Database/Esqueleto/Internal/Internal.hs#L809-L846

Here's the general flow of how you can make changes:

1. Fork the repo and clone it.
2. Make the change locally.
3. Run stack build --fast and stack haddock --fast to ensure the library and documentation builds correctly.
   • The Haddock html file will be created in the .stack-work/ directory within the repo after running the build, which you can open to verify that the docs are correct
4. You can add the local copy of the project as a dependency to your own project to verify the new functions work, and/or update the tests.
5. Push your changes back up to your fork and make a pull request into this repo.

[belevy]

This function isn't standardized across implementations so the function should go into the backend specific modules