hash_to_curve(msg)

Input: msg, an arbitrary-length byte string.
Output: P, a point in G.

Steps:
1. u = hash_to_field(msg, 2)
2. Q0 = map_to_curve(u[0])
3. Q1 = map_to_curve(u[1])
4. R = Q0 + Q1              # Point addition
5. P = clear_cofactor(R)
6. return P