Add effect module

[andrejewski]

This PR adds helpers for effects. I am hesitant to add anything but the essentials to this framework's API surface. These effect helpers I believe are foundational to building decomposed apps and assembling them together. To ask users to write these helpers by hand has problems:

• Errors, as they may not be tested with the scrutiny I test with
• Inconsistency, as there are many ways to implement the happy path without the edge cases
• Familiarity is lost, as everyone lacks a shared foundation moving between re-created helpers

For these reasons, I believe this module is worth adding.