Remove operations on full-group elements from the public API

[str4d]

G1 and G2 are prime-order subgroups, but they currently have several public APIs that don't apply to prime-order groups:

• G1Affine::is_torsion_free
• G1Affine::is_on_curve
• G1Projective::clear_cofactor
• G1Projective::is_on_curve
• Similarly for G2Affine and G2Projective.

The is_* APIs are all documented as "Always returns true unless the unchecked APIs were [mis]used". Meanwhile, clear_cofactor (added in #18) is defined as mapping elliptic curve points to elements of G_{1,2}, but the type it is defined on is already an element of G_{1,2}.

These APIs were implemented because of the G*Affine::from_[un]compressed_unchecked APIs, which can technically be used to load full-group elements, but doing so is explicitly breaking the requirements set by the *_unchecked APIs, so this shouldn't be a supported case.

We should make the above APIs private (all except G*Projective::is_on_curve are used internally), and then consider on a case-by-case basis how to handle full-group elements (which in general we do not want to expose or support).