add signal builders interface

[1024bees]

this is a first pass PR at adding "derived" signals in a fairly well defined way, in both python and rust

in python, we have a working test for the "slice" case -- while we could support this with custom code in rust, i think its a decent exercise for showing how we can use python logic to create a new signal

in rust, we have a bunch of unused code -- the SimpleLazy example is unused in tests, and the ClockedSignal has an ignored test that isnt fully implemented.

overall, i think the python code is in a much better state than the rust code (the test isn't fully implemented), and if we want to merge it quickly then we should probably remove the derived_signals rust code for now

look at the tests for examples of these apis

[ekiwi]

This looks great! However, I would like to simplify the review. Since you yourself already indicated that the Rust stuff isn't quite done, how about you isolate all the changes that are necessary for the functionality demonstrated in the Python test_slice() function and make a PR with that. Then we can discuss and work on the Rust stuff as a separate issue.

[ekiwi]

@1024bees feel free to ping me if you need help with this! Thanks for working on the python bindings.

[1024bees]

doing a little bit of wandering on it :) pipecleaning it with https://github.com/1024bees/dang, will shoot to have it wrapped up this weekend knowing that youre Looking.

[ekiwi]

doing a little bit of wandering on it :) pipecleaning it with https://github.com/1024bees/dang, will shoot to have it wrapped up this weekend knowing that youre Looking.

Perfect! Please don't feel any pressure vis-à-vis timeline. The one thing I am asking for is that you try to keep your PRs relatively small.