Support for basic (nested) looping

This relates to implementing the ability to perform looping constructs as explored in the hackathons [See: https://gitlab.com/quantify-os/quantify/-/wikis/201023-Hackthon-notes].

We still need to converge on a design, but a design should include

• A convenient interface (such as that using context managers).
• A design for the data structures, possibly involving nesting.
• Take forward compatibility into account, thinking also about basic logic (if/else, while, and case statements).

Relates to "Implement loops and classical logic in scheduler"