Rename candidNumberArrayToBigInt (#697)

# Motivation

Since the utility `candidNatArrayToBigInt` will be used for converting
not only the `Nat` type but also `Int`, its name should be more generic.

# Changes

- Rename `candidNatArrayToBigInt`  to `candidNumberArrayToBigInt`.

# Tests

Pass.

# Todos

- [ ] Add entry to changelog (if necessary).
Not necessary