refactor: rename MaybeRelocatable to SegmentValue

[zmalatrax]

MaybeRelocatable represents the value a memory cell can take during a run of the VM, before the relocation phase near the end.
MaybeRelocatableis essentially an union type: Relocatable | Felt.

But the name MaybeRelocatable doesn't provide more sense, details or anything to it. Therefore, it should renamed to something more meaningful, such as SegmentValue. The same way, the attribute data in the class Memory should be replaced by values.
All the related occurences of MaybeRelocatable and memory.data in variable names, methods etc. should be renamed consequently.