Refactor Periodic columns handling in IR and codegen

1. Based on @bobbinth's comments on PR #40 [here](https://github.com/0xPolygonMiden/air-dsl/pull/40#discussion_r1007268169), it might be better to change the periodic columns to `Vec>` instead of `Vec>` 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](https://github.com/0xPolygonMiden/air-dsl/pull/40#discussion_r1007277015).

> 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.