Refactor Periodic columns handling in IR and codegen

[tohrnii]

1. Based on @bobbinth's comments on PR Add codegen for periodic columns #40 here, it might be better to change the periodic columns to Vec<Vec<Felt>> instead of Vec<Vec<u64>> at IR level as it might be helpful in the future when we may want to evaluate constraints at a random point..

2. Also we could refactor the to_string() method for periodic columns proposed by @bobbinth here.

A thought for the future: instead of doing conversions from u64 to Felt here, we could do the following:

1. Define static arrays for all periodic columns such that these arrays already contain Felt's.
2. Inside get_periodic_column_values() convert these arrays to vectors (which should be just memory copy operations) and return.
   The performance benefit of the above is probably very minor - so, definitely not a high priority item.