feat: customize Cslib/Foundations/Data/OmegaList/*

[ctchou]

In the previous PR #80, ωList is just a copy of Stream' with suitable name changes. Now we make three major modifications:

1. We make it clear that ωList α is really an abstraction of ℕ → α. In particular, for s : ωList and n : ℕ, the n-th elements of s is now obtained by s n, not s.get n or get s n. Indeed, we have got rid of the API function get altogether.
2. The infix notation for appendωList is changed from++ₛ to ++ₗ (the last letter is a lowercase "L" subscript). This is to avoid conflict with Stream'.
3. A new API function extract, which is the analogue of List.extract, and a number of theorems involving it are added.

[ctchou]

Rebase on the current "main".