Compile fix. I don't have GSSAPI
Internal types only: enum -> macros.
Nettle.ECC_Curve: Improved documentation about point_mul().

Added note that point_mul() validates that the coordinate
is valid for the curve.
SSL.Cipher [ecdh]: Added note about point validation.

There are attacks against ECDH/ECDHE that are based on providing
points that aren't on the negotiated curve, so add an explicit
note that the received points are validated in point_mul().
Fix Stdio.File compat code.
Remove superfluous legacy compat support prior to 7.8.
Remove superfluous legacy compat support prior to 7.8.
Remove superfluous legacy compat support prior to 7.8.
Remove superfluous legacy compat support prior to 7.8.
Use add_hint
Use read_hint. It has to go through Gmp.mpz until all relevant interfaces are converted into int.
Add 8.0 compat and remove references to 7.0 and 7.2 compat.