gh-37092: Adding methods .x() and .y() for EllipticCurvePoint

    
Added the more natural notation P.x() and P.y() to access respectively
the x and y coordinates of an EllipticCurvePoint. The behaviour mimics
P.xy() and aims to replace P.xy()[0] and P.xy()[1].
Multiple instances of P.xy()[0] and P.xy()[1] are also replaced in
related modules.

#sd123

Done with @gioella
    
URL: #37092
Reported by: Riccardo Invernizzi
Reviewer(s): Lorenz Panny, Riccardo Invernizzi