chore: replace phi by f to avoid potential conflict with Euler totient function

[ctchou]

I just noticed that the Greek letter φ is used to denote the Euler totient function in the Nat namespace. Since the theory developed in Cslib/Foundations/Data/Nat/Segment.lean is also in the Nat namespace, its use of φ as a function name is potentially dangerous. This patch replaces all occurrences of φ by f and all previous occurrences of f by g.