Add a method to map values to points on the curve

[xevisalle]

In this PR, we implement a method map_to_point to map a u64 input value to a point on the curve. Likewise, we implement a unmap_from_point method, to be able to recover the original value.

The mapping we implemented is probabilistic, meaning that trying to find a correct map can require several tries, until finding a real point on the curve, that lies also on the correct subgroup.

To achieve such an outcome, we craft a 32-bytes representation of a point as follows:
target_point = (random_value[31..8] || u64_input_value)

given that, we check if this point is valid or not. If that's not, we keep iterating and checking again, by adding u64_size + 1 each iteration.

[marta-belles]

LGTM but it would be good if a dev also takes a look at it