Start cleaning up oct2point and point2oct

The SEC 1 standard defines various ways of encoding an elliptic curve
point as ASN.1 octet string. It's also used for the public key, which
isn't an octet string but a bit string for whatever historic reason.
The public API is incomplete and inconvenient, so we need to jump
through a few hoops to support it and to preserve our own sanity.

Split a small helper function out of ec_GFp_simple_point2oct() that
checks that a uint8_t represents a valid point conversion form. It
supports exactly the four possible variants and helps translating
from point_conversion_form_t at the API boundary.

Reject the form for the point at infinity since the function has
historically done that even for the case that the point actually is
the point at infinity.

ok jsing

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