`OpenSSL::PKey::EC::Point` should be able to get and set affine coordinates

It is possible using a little magic to create a point with a raw x and y:

   def self.from_compressed_point(group, x, is_even)
      x_value = x.to_s(2).rjust(32, "\x00")
      prefix = is_even ? "\x02" : "\x03"

      encoded = OpenSSL::BN.new [ prefix, x_value ].join, 2
      OpenSSL::PKey::EC::Point.new(group, encoded)
    end

One can similarly use some magic to get uncompressed points and turn them into two OpenSSL::BN instances

It would be better if we supported this without using compressed point form magic, like so (RBS signature with hypothetical overload):

class OpenSSL::PKey::EC::Point
  # Since the OpenSSL operation returns both
  def affine_coordinates -> [ OpenSSL::BN, OpenSSL::BN ]
  
  # Convenance methods for above
  attr_reader :x, :y -> OpenSSL::BN

  #  Choice 1 - Using Bool for Y
  def initialize : (group: OpenSSL::PKey::EC::Group, x: OpenSSL::BN, even: Boolean) -> OpenSSL::PKey::EC::Point

  # Choice 2 - Using a symbol for Y
  def initialize : (group: OpenSSL::PKey::EC::Group, x: OpenSSL::BN, y:[ :even, :odd ]) -> OpenSSL::PKey::EC::Point

  # Using X and Y directly
  def initialize : (group: OpenSSL::PKey::EC::Group, x: OpenSSL::BN, y: OpenSSL::BN) -> OpenSSL::PKey::EC::Point
end

I've intentionally omitted mutating the X and Y of a point after creation. Because all of these have a different aridity they will not conflict. Similarly adding Jacobian would create an aridity == 4.

The big question is for choice 1 or 2 when creating an X and which Y value.

No files or tests are named. Start by reviewing the existing OpenSSL::PKey::EC::Point API and the compressed and uncompressed point examples in the issue. Resolve the proposed constructor and coordinate-access design with maintainers, then define tests showing affine-coordinate retrieval and point construction work as intended.