Change sk and pk suffixes

[m1sta]

I appreciate that the sk and pk suffixes are used for consistency with the nacl documentation however I think there might be a risk that new developers will think 'sk' equates to 'shared key' and 'pk' equates to 'private key' (contrary to the documentation). This feels relevant given that one of the goals of nacl (as I understand it) is to reduce the impact of inexperienced developers using crypto.