deprecate old macros, deprecate mp_rand_digit

[minad]

Some first deprecations in the public API. Desired? See also #243

[sjaeckel]

Not 100% sure if mp_rand_digit() should be deprecated but the rest LGTM

[minad]

I think mp_digit exposure in the public api should be reduced - mp_digit should be seen more as a implementation detail, since it varies greatly depending on the target platform (8, 16, 28, 31, 32, 60 bits etc).

mp_rand_digit is one such function which works with mp_digit and therefore a candidate for deprecation. I added it here since I think it probably doesn't have many consumer under the library users.

Also note: The way I am proposing regarding deprecations always allows us to go back if many downstream users come and tell us that we deprecated something they needed. Just undeprecate and move on in that case instead of removing the deprecated api.